Epidemiology lecture notes.ppt

8/18/2019 Epidemiology lecture notes.ppt

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Epidemiology

By

Dr.Imtiaz Husain


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1. Introduction to Epidemiology


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Definitions

Health: A state of complete physical, mental and social well !ein" and not merely the a!sence of disease orinfirmity #$H%,1&'()

Disease: A physiolo"ical or psycholo"ical dysfunction

Illness: A su!*ecti+e state of not !ein" well

Sickness: A state of social dysfunction


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Definitions…

Public health

he science - art of P reventing disease, prolonging life,

promoting health & efficiency throu"h or"anized community effort #$inslow, 1&2)


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omponents of the definition

1.Study: ystematic collection, analysis andinterpretation of data

pidemiolo"y in+ol+es collection, analysis andinterpretation of health related data

Epidemiology is a science


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omponents…

2. Frequency: the num!er of times an e+ent occurs

pidemiolo"y studies the num!er of times adisease occurs

It answers the 4uestion How many5

Epidemiology is a quantitative science


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omponents…

3. Distribution: Distri!ution of an e+ent !y person, place and time

pidemiolo"y studies distri!ution of diseases

It answers the 4uestion who, where and when?

Epidemiology describes health events


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omponents…

'. Determinants: 6actors the presence7a!sence ofwhich affect the occurrence and le+el of an e+ent

pidemiolo"y studies what determines healthe+ents

It answers the 4uestion how and why5

Epidemiology analyes health events


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omponents…

/. Diseases ! other health related events

pidemiolo"y is not only the study of diseaseshe focus of pidemiolo"y are not only patients

It studies all health related conditions

Epidemiology is a broader science


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omponents…

. #pplication

pidemiolo"ical studies ha+e direct and practicalapplications for pre+ention of diseases - promotion of health

pidemiolo"y is a science and practice

Epidemiology is an applied science


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History of Epidemiology

Se!en land marks in the history of Epidemiology

1. Hippocrates #'0B8): n+ironment - human !eha+iors affects health

$. %ohn &raunt #1002): 9uantified !irths, deathsand diseases

'. (ind #1'): cur+y could !e treated with freshfruit


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History…

'. )illiam Farr #1(3&): sta!lished application of+ital statistics for the e+aluation of health

pro!lems/. %ohn Snow #1(/'): tested a hypothesis on the

ori"in of epidemic of cholera

0. #le*ander (ouis

#1(2): ystematized applicationof numerical thinin" #4uantitati+e reasonin")


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History…

. +rad"ord Hill #1&3): u""ested criteria foresta!lishin" causation

Epidemiological thought emerged in - +/

Epidemiology "lourished as a discipline in 10s


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Scope of Epidemiology

riginally, pidemiolo"y was concerned within+esti"ation - mana"ement of epidemics ofcommunica!le diseases

(ately, pidemiolo"y was e;tended to endemiccommunica!le diseases and noncommunica!le

infectious diseases

2ecently, pidemiolo"y can !e applied to all diseases and other health related e+ents


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Purpose"use of Epidemiology

he ultimate purpose of pidemiolo"y is pre+ention of diseases and promotion of health

How?

1. lucidation of natural history of diseases

2.Description of health status of population3. sta!lishin" determinants of diseases

'. +aluation of inter+ention effecti+eness


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#ypes of Epidemiology

3wo ma*or cate"ories of pidemiolo"y 1.Descriptive Epidemiology

Defines "requency and distribution of diseasesand other health related e+ents

Answers the four ma*or 4uestions: how many,who, where, and when?


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#ypes…

2. #nalytic Epidemiology

Analyses determinants of health pro!lems

Answers two other ma*or 4uestions: how5 andwhy5


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$asic Epidemiological assumptions

1.Human diseases doesn=t occur at random or !ychance

2. Human diseases ha+e causal and preventive factors


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$asic features of Epidemiology

1. tudies are conducted on human population

2. It e;amines patterns of e+ents in people

3. 8an esta!lish causeeffect relationship withoutthe nowled"e of !iolo"ical mechanism

'. It co+ers a wide ran"e of conditions

/. It is an ad+ancin" science


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%. ommunicable disease

Epidemiology


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Disease causation

#he cause of a disease

An e+ent, a condition or a characteristic thatcomes !efore the disease and without which thedisease wouldn=t occur


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#heories…

wentieth century theories

1.


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&ecessary 's Sufficient

&ecessary: the disease will not occur without the presence of the factor

;ample: @yco!acterium B for B

Sufficient: the presence of the factor always resultin disease;ample: a!ies +irus for ra!ies


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Etiology of a disease

he sum of all factors contri!ution to theoccurrence of a disease

A"ent factors CHost factors Cn+ironmentalfactors tiolo"y of a disease


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Disease models

How do diseases de+elop5

hree !est nown models

1.Epidemiological triangle

he interaction of an a"ent and host in an

appropriate en+ironment results in disease


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Disease models…

2. )eb o" causation

8omple; interaction of factors results in disease

3. )heel model

he hu! #host) ha+in" a "enetic mae up as its core,

surrounded !y an en+ironment schematically di+idedin to !iolo"ical, physical and social


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&atural history…

2. Stage o" sub4clinical disease

?resence of patho"enic chan"es #!iolo"ical onset)

>o disease manifestations

3. Stage o" clinical disease

?resence of si"n and symptoms #clinical onset)

'. Stage o" recovery, disability,or death


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(e!els of disease pre!ention

hree ma*or le+els of disease pre+ention

1. 5rimary prevention

ar"eted at healthy people

%!*ecti+es are ?romotion of health

?re+ention of e;posure and?re+ention of disease


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(e!els of disease…

2. Secondary prevention

ar"eted at sic indi+iduals

%!*ecti+e is to stop or slow the pro"ression ofdisease and to pre+ent or limit permanent dama"e

throu"h early detection - treatment


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(e!els of disease…

3. 3ertiary prevention

ar"eted at people with chronic diseases -disa!ilities that can=t !e cured

%!*ecti+e is to pre+ent further disa!ility or deathand to limit impacts of disa!ility throu"hreha!ilitation


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#he agent

?ossi!le outcomes of e;posure to an infectious a"ent

Infection: in+asion - multiplication in the host

Infectivity: the proportion of e;posed who !ecomesinfected

Infection rate Infected7e;posedDisease: A clinically apparent infection


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#he agent…

Pathogenicity:the proportion of infected whode+elop clinical disease

Clinical-to-Subclinical ratioirulence: the proportion clinical cases resultin" inse+ere clinical disease

Case fatality & hospitali!ation rate

Immunogenecity:the infection=s a!ility to producespecific immunity


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)eser!oir 's arrier

2eservoir

An or"anism or ha!itat in which an infectiousa"ent normally li+es, transforms, de+elops and7or

multiplies

/arrier

A person who doesn=t ha+e apparent clinicaldisease, !ut is a potential source of infection toother people


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#ypes of carriers

1.6ncubatory carriers: transmits the disease durin"incu!ation period

;ample: @easles, mumps

$. /onvalescent carriers: transmits the diseasedurin" con+alescent period

;ample: yphoid fe+er


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#ypes of carriers…

3. #symptomatic carriers: transmittin" the diseasewithout showin" manifestations

;ample: polio, Amoe!iasis

'. /hronic /arriers: transmittin" the disease forlon" time7indefinite transmission

;ample: Eiral hepatitis, typhoid fe+er


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Importance of carriers

1. 7umber carriers may outnum!er cases

$. Di""iculty in recognition carriers don=t now that

they are infected'. 8obility carriers are mo!ile, cases are restricted

. /hronicity carriers reintroduce infection andcontri!ute to endemicity


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*odes of disease transmission

1.Direct transmission

Direct contact : physical contact with !ody part ofinfected person: ouchin", issin",!itin",se;

;ample: HIE Direct pro9ection: pro*ection of sali+a droplets whilecou"hin", sneezin",spittin",talin",sin"in" etc ;ample: 8ommon cold

3ransplacental : ransmission from mother to fetusthrou"h the placenta ;ample: yphilis


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*odes of disease…

2. Indirect transmission

:ehicle4borne: transmission throu"h

inanimate o!*ects7nonli+in" su!stances e."HIE !y needles

#ir4borne: transmission !y dust or dropletnuclei


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*odes of disease…

:ector4borne: infectious a"ent is con+eyed !y anarthropod to host

+iological : there is multiplication and7orde+elopment in the +ector

Salivarian: In*ects infected sali+a e." mos4uito

Stercorarian: infects !y infected feaces e." louse

8echanical : simple transfer without !iolo"icalsta"es in the +ector e." flies


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Importance of mode of transmission

F A disease often has se+eral modes of transmission F It is important to distin"uish !etween the

predominant mode of transmission and those ofsecondary importance

F Identifyin" primary and secondary modes oftransmission is important to identify mosteffecti+e pre+ention and control measures


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Herd immunity

It is host resistance at a population le+el

It is defined as the resistance of a community

#"roup) to in+asion and spread of an infectiousa"ent, !ased on immunity of hi"h proportions ofindi+iduals in the community.

It has implications on +accination pro"rams


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#ime course of an infectious disease

5re4patent period : !etween !iolo"ical onsetand first sheddin"

6ncubation period : !etween !iolo"ical onsetand clinical onset

/ommunicable period : time durin" whicha"ent is !ein" shed


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#ime course of…

(atent period : !etween reco+ery and relapse inclinical disease

/onvalescent period : !etween reco+ery and timewhen sheddin" stops

&eneration period : !etween e;posure7infection andma;imum communica!ility of e;posed host


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-actors +hich influence the de!elopment of

disease

train of the a"ent

Dose of the a"ent

oute of infectionHost a"e, nutritional status, immune status

Influence of treatment

Influence of season


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. *easures of disease

occurrence


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/hat are measures of disease occurrence0

hese are measurements of thefre4uency7ma"nitude7amount

of disease

in populations


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Ho+ do +e measure diseases0

6our quantitative descriptorsF >um!ers

F atiosF ?roportionsF ates


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Descriptors

7umbers: Gse of actual num!er of e+ents

e." 1 cases of B in community A

2atios: 9uantifies the ma"nitude of one occurrence, in relation to another e+ent J as 7J

e." atio of B cases in community A to B is1:1


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/hich community is more affected0

8ommunity A has 1 cases of disease

and

8ommunity B has 1 cases of disease ,

which community is more affected5


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/hen +e call..

F $hen we call a measure a ratio, we mean a non proportional ratio

F $hen we call a measure a proportion, we mean a proportional ratio that doesn=t measure an e+ento+ertime

F $hen we call a rate, we mean a proportional ratiothat does measure an e+ent in a populationo+ertime


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#ypes of rates

1. /rude rates: Apply to the total population in a"i+en area

$. Speci"ic rates: Apply to specific su!"roups inthe population #a"e, se; etc) or specific diseases

'. Standardied rates: used to permit comparisons of

rates in population which differ in structure #e."a"e structure)

wo methods of standardization:Direct, indirect


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*orbidity rates

@or!idity rates are rates that are used to 4uantifythe ma"nitude7fre4uency of diseases

wo common mor!idity rates

Incidence rates#8umulati+e incidence, incidencedensity)

?re+alence #?eriod pre+alence, point pre+alence)


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Incidence rate

he proportion of a population that de+elops a

disease o+ertime

he ris7pro!a!ility of an indi+idual de+elopin" adisease o+ertime

he rapidity with which new cases of a diseasede+elop o+ertime

he proportion of unaffected indi+iduals who ona+era"e will contract the disease o+ertime


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Practical challenges in measuring incidence

rate

1. Identification of population at ris

?opulation at ris constitutes all those free ofthe disease and suscepti!le to it

2. ?opulation is not static7it fluctuates7as a result of !irths, deaths and mi"ration

3. ?eople are at ris only until they "et the diseaseand then no more at ris


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Practical solution to the challenges

1. Gse the total population as a denominator

his "i+es an estimate of the incidence rate and

not the actual incidence rate

2. Gse person4time at ris

Incidence densitynum!er of new cases of adisease o+er a specified period7persontime atris


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Pre!alence rate

It measures the proportion of a population with adisease durin" a specified period or at a point intime

wo types

1. ?oint pre+alence rate

2. ?eriod pre+alence rate


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Point pre!alence rate

@easures the proportion of a population witha disease at a point in time

?oint pre+alence rateAll persons with adisease at a point in time7otal population

It is not a rate, !ut a true proportion


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Period pre!alence rate

@easures the proportion of a population witha disease in a specified time period

?eriod pre+alence rateAll persons with adisease o+ertime period7A+era"e#mid

year)population in the same period


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Incidence 's pre!alence

Incidence rate considers only new cases of a disease

?re+alence rate considers all #new C old) cases of a disease

Incidence rate considers population at ris as a denominator

?re+alence rate considers total population as a denominator

Incidence - period pre+alence rates re4uire follow up studies

?oint pre+alence rate re4uires cross sectional study


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)elationship bet+een pre!alence

incidence rates

?re+alence L incidence

?re+alence L A+era"e duration

?re+alence L Incidence A+era"e duration

An increase in pre+alence rate may not necessarily !e due to an increase in incidence rate, it coulddue to an increase in a+era"e duration of a diseasedue to decrease in death and7or reco+ery rates


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*ortality rates

hese rates measures ma"nitude of deaths in acommunity

ome are crude lie the crude death rate

%thers are causespecific mortality rate

ome others are ad*usted lie standardizedmortality ration


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ommon *ortality rates

F 8rude death rateF A"especific mortality rateF e;specific mortality rateF 8ausespecific mortality rateF ?roportionate mortality ratioF 8ase fatality rate

F 6etal death rate

F ?erinatal mortality rateF >eonatal mortality rateF Infant mortality rateF 8hild mortality rateF Gnderfi+e mortality

rate

F @aternal mortality ratio


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2. *easures of association


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%3% table

Disease

Jes #C) >o #C) otal

;posure Jes #C) a ! aC!

>o #C) c d cCd

otal aCc !Cd aC!CcCd


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ells

A ;posed, and diseased

B ;posed, >ot diseased8 >ot e;posed, diseased

D >ot e;posed, >ot diseased


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#otals

@ar"inal totals

aC! ;posed

cCd >one;posed aCc Diseased

!Cd >ondiseased


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hi4s5uare statistics

8his4uare tests whether there is an association !etween two cate"orical +aria!les

Ho: here is no association !etween row - column+aria!les

Ha: here is an association !etween row and column+aria!les

8his4uare statistic has a de"ree of freedom #r1)#c1),where r is num!er of rows - c num!er of columns


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hi4S5uare… M 2 N #% )2

%: %!ser+ed cells: ;pected cells

;pected +alue #ow total)#8olumn total)


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Importance of hi4s5uare

If the calculated chis4uare +alue is "reater than

the critical or ?O./ we say that there isassociation

8his4uare statistics tells only whether there is

association. It doesn=t tell us how much stron" anassociation is.


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)elati!e risk 6))7

;presses ris of de+elopin" a diseases in e;posed"roup #a C !) as compared to none;posed "roup #c C d)

Incidence #ris) amon" e;posed Incidence #ris) amon" none;posed

a7#aC!) c7#cCd)


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Interpretation of relati!e risk

$hat does a of 2 mean5

is in e;posed is in none;posed

of 2 means

is in e;posed2 is in none;posed

hus a relati+e ris of 2 means the e;posed "roup is twotimes at a hi"her ris when compared to none;posed


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Strength of association

In "eneral stren"th of association can !econsidered as:

Hi"h if P3

@oderate if is !etween 1./ - 2.&

$ea if is !etween 1.2 - 1.'


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8dds ratio 68)7

%dds ratio is the ratio of odds of e;posure amon"diseased to odds of e;posure amon" nondiseased

%dds of an e+ent is the ratio of pro!a!ility of thee+ent to its complement

%dds?#)7?#=)?#)7#1?#))


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8dds ratio…%dds of e;posure amon" e;poseda7c%dds of e;posure amon" nondiseased!7d

% %dds of e;posure amon" diseased

%dds of e;posure amon" nondiseased

% #a7c)7#!7d)

% ad7!c #it is also called crossproduct ratio)

Interpretation of % is the same as that of


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8dds ratio…

can !e !est estimated !y % if the followin"conditions are fulfilled

1. 8ontrols are representati+e of "eneral population2. elected cases are representati+e of all cases

3. he disease is rare


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,ttributable )isk 6,)7

A indicates how much of the ris is due to7attri!uta!le7 to the e;posure

9uantifies the e;cess ris in the e;posed that can !eattri!uta!le to the e;posure !y remo+in" the ris of thedisease occurred due to other causes

A is #incidence) in e;posed is #incidence) in none;posed

A Qa7#aC!)R 7 Qc7#cCd)))R Attri!uta!le ris is also called ris difference


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Interpreting ,)

$hat does attri!uta!le ris of 1 mean5

1 of the e;posed cases are attri!uta!le to thee;posure

By remo+in" the e;posure one can pre+ent 1cases from "ettin" the disease


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,ttributable risk percent 6,)97

stimates the proportion of disease amon" thee;posed that is attri!uta!le to the e;posure

he proportion of the disease in the e;posed that can !e eliminated !y eliminatin" the e;posure

AK #is in e;posed is in none;posed)1K is in none;posed


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Interpretation of ,)9

$hat does AK of 1K mean5

1K of the disease can !e attri!uted to the e;posure

1K of the disease can !e eliminated if we a+oid thee;posure


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Population ,ttributable )isk 6P,)7

stimates the rate of disease in total populationthat is attri!uta!le to the e;posure

?A is in population is in une;posed

?A A pre+alence rate of e;posure


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Population attributable risk percent 6P,)

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stimates the proportion of disease in the study population that is attri!uta!le to e;posure and thuscould !e eliminated if the e;posure were eliminated

?AK is in population is in une;posed

is in population


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Possible outcomes in studying the

relationship bet+een eposure disease

1. >o association 1 A2. ?ositi+e association P1 AP

3. >e"ati+e association O1 #fraction) AO #>e"ati+e)


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)isk 's Pre!enti!e factors

A ris; "actor is any factor positi+ely associated witha disease #P1)

It is associated with an increased occurrence of a

disease A preventive "actor is any factor ne"ati+ely

associated with a disease #O1) It is associated with a decreased occurrence of a

diseaseis and pre+enti+e factors may


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,ssociation 's ausation

he e;istence of an association doesn=t itselfconstitute a proof of causation

An o!ser+ed association could !e a fact or anartifact

Hence, an association is a necessary !ut not asufficient condition for causation


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Possible eplanations for obser!ed

association

1. 8hance

2. Bias

3. 8onfoundin"'. e+erse causation

/. eciprocal causation

0. 8auseeffect relationship


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,ccuracy of measurement

Accuracy Ealidity C ?recision

Ealidity is the e;tent to which a measured+alue actually reflects truth

here are two types of +alidity Internal +alidity ;ternal +alidity


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#ypes of !alidity

Internal !alidity:

Is the de"ree to which a measured +alue is true withinthe sample

Eternal !alidity:

Is the e;tent to which a measured +alue apply !eyond

the sample

his is related to "eneraliza!ility


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)ole of chance

he role of chance as an alternati+e e;planationfor an association emer"es from samplin"+aria!ility

+aluation of the role of chance is mainly thedomain of statistics and in+ol+es

1. est of statistical si"nificance

2. stimation of confidence inter+al


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1. #est of statistical significance

?+alue 4uantifies the de"ree to which chanceaccounts for o!ser+ed association

?+alue is the pro!a!ility of o!tainin" a result atleast as e;treme as the o!ser+ed !y chance alone

?O./ indicates statistical si"nificance for medicalresearch


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#est of statistical…

A +ery small difference may !e si"nificant if youha+e lar"e sample

A lar"e difference may not achie+e statisticalsi"nificance is you ha+e small sample

%ne can=t mae a definite decision !ased on p+alueonly


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)ole of bias

Bias is any systematic error in the desi"n, conduct oranalysis of an epidemiolo"ic study that results in anincorrect estimate of association !etween e;posureand disease

Gnlie chance !ias can=t !e statistically e+aluated

here are two ma*or types of !ias

1. election !ias2. Information !ias


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Selection bias

Any systematic error that arises in the process ofidentifyin" the study population

It affects the representati+eness of the study

It occurs when there is a difference !etween sampleand population with respect to a +aria!le

;amples of selection !ias:1. Dia"nostic !ias2. Eolunteer !ias

3. >onresponse !ias

'. oss to followup !ias


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/ays to minimi=e bias

1. 8hoose study desi"n carefully

2. 8hoose o!*ecti+e rather than su!*ecti+e

outcomes3. Blind inter+iewers whene+er possi!le

'. Gse close ended 4uestions whene+er possi!le

/. 8ollect data on +aria!les you don=t e;pect todiffer !etween two "roups


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)ole of confounding8onfoundin" refers to the mi;in" of the effect of ane;traneous +aria!le with the effect of the e;posureand disease of interest

8haracteristics of a confoundin" +aria!le1. Associated with disease in a!sence of e;posure2. Associated with e;posure !ut not as a conse4uence of e;posure

3. he fre4uency of the confoundin" +aria!le +ary !etween the"roups that are compared

;ample: In association !etween smoin" and lun" cancer alcoholdrinin" suspected as a confoundin"


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Effect of confounding

otally or partially account for the apparent effect

@as an underlyin" true association

e+erse the actual direction of association


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ontrol of confounding !ariables

Durin" desi"nin" sta"e: andomization estriction

@atchin"

Durin" analysis sta"e tandardization tratification7poolin" @ulti+ariate analysis

riteria to asses the strength of e!idence


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riteria to asses the strength of e!idence

for cause and effect relationship

%!ser+ational studies ha+e many !iases andconfoundin". ;perimental studies if properlydone can show causeeffect relationship. But they

are not usually feasi!le due to ethical issues

In the a!sence of an e;perimental trail, thefollowin" criteria #Bradford Hill criteria) are usedto asses the stren"th of e+idence for a cause andeffect relationship


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riteria to asses the strength…

1. Strength o" association: he stron"er theassociation the more liely it is causal

2. /onsistency o" association: he more consistent,the more liely it is causal

3. Speci"icity o" association: If sin"le e;posure

lined to sin"le disease more liely

'. 3emporal relationship: he e;posure must come !efore the disease


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>. Epidemiologic Study Designs


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Study design

tudy desi"n is the arran"ement of conditions for

the collection and analysis of data to pro+ide themost accurate answer to a 4uestion in the mosteconomical way.


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#ypes of Epidemiologic study designs

I. Based on o!*ecti+e7focus7research 4uestion

1. Descripti+e studies Descri!e: who, when, where - how many

2. Analytic studies Analyse: How and why


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#ypes…

II. Based on the role of the in+esti"ator

1. %!ser+ational studies he in+esti"ator o!ser+es nature >o inter+ention

2. Inter+ention7;perimental studies In+esti"ator inter+enes He has a control o+er the situation


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#ypes…

III. Based on timin"

1. %netime #onespot) studies 8onducted at a point in time An indi+idual is o!ser+ed at once

2. on"itudinal #6ollowup) studies 8onducted in a period of time Indi+iduals are followed o+er a period of time


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#ypes…

IE. Based on direction of followup7data collection 1. ?rospecti+e

8onducted forward in time

2. etrospecti+e

8onducted !acward in time


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#ypes…

EI. Based on study settin"

1. 8ommunity!ased studies 8onducted in communities

2. Institution!ased studies 8onducted in communities

3. a!oratory!ased studies 8onducted in ma*or la!oratories


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#ypes…

EII. tandard classification

1. 8rosssectional studies2. 8asecontrol studies

3. 8ohort studies

'. ;perimental studies


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ross4sectional studies In this study desi"n information a!out the status of an

indi+idual with respect to presence7a!sence ofe;posure and diseased is assessed at a point in time.

8rosssectional studies are useful to "enerate ahypothesis rather that to test it

6or factors that remain unaltered o+ertime #e.". se;,race, !lood "roup) it can produce a +alid association


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ross4sectional…

Ad+anta"es of crosssectional studies

F ess time consumin"F ess e;pensi+eF ?ro+ides more informationF Descri!es wellF


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ross4sectional…

imitations of crosssectional studies

F Antecedentconse4uence uncertainty U8hicen or e"" dilemmaVF Data dred"in" leadin" to inappropriate

comparison

F @ore +ulnera!le to !ias

i l


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ross4sectional…

ypes of crosssectional studies

1. in"le crosssectional studies Determine sin"le proportion7mean in a sin"le population at

a sin"le point in time2. 8omparati+e crosssectional studies

Determine two proportions7means in two populations at asin"le point in time

3. imeseries crosssectional studies Determine a sin"le proportion7mean in a sin"le population

at multiple points in time

ti l


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ross4sectional…

In "eneral, crosssectional studies are implest to conduct

8ommonest to find east useful to esta!lish causation

t l t di


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ase4control studies

F u!*ects are selected with respect to the presence#cases) or a!sence #controls) of disease, and thenin4uiries are made a!out past e;posure

F $e compare diseased #cases) and nondiseased#controls) to find out the le+el of e;posure

F ;posure status is traced !acward in time

t l


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ase4control…

teps in conductin" casecontrol studiesI. Define who is a case

sta!lish strict dia"nostic criteria

All who fulfil the criteria will !e Ucase population hose who don=t fulfil will !e Ucontrol populationV

II. elect a sample of cases from case population

his sample must !e representati+e of the case population

t l


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ase4control…

ources of cases

1. Hospitals #Health institution) 8ostless Biasmore

2. ?opulation #8ommunity)

8ostmore Biasless

t l


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ase4control…

III. elect controls from a control population hould !e representati+e of control population hould !e similar to cases e;cept outcome hould !e selected !y the same method as cases

ources of controls1. Hospital #Health institution) controls

eadily a+aila!le

ow recall !ias @ore cooperati+e

t l


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ase4control…

Howe+er, hospital controls are ess representati+e @ore confoundin"

2. ?opulation #community) controls @ore representati+e ess confoundin"

8ostly and time consumin" @ore recall !ias ess cooperati+e

t l


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ase4control…

IE. @easure the le+el of e;posure in cases - controls e+iew or inter+iew for e;posure status Gse same method for case and controls

E. 8ompare the e;posure !etween cases - controls ?repare 22 ta!le

8alculate % ?erform statistical tests

ase control


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130

ase4control…

ypes of casecontrol studies

I. Based on case identification

1. etrospecti+e casecontrol Gses pre+alent cases Increased sample size Difficult to esta!lish temporal se4uence Gseful for rare outcomes

ase control


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ase4control…

2. ?rospecti+e casecontrol Gses incident cases sta!lish temporal se4uence

ecall is not a serious pro!lem ecords are easily o!taina!le


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ase control


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ase4control…

Ad+anta"es of casecontrol studies

F%ptimal for e+aluation of rare diseasesF ;amines multiple factors of a sin"le disease

F 9uic and ine;pensi+eF elati+ely simple to carry outF


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ase4control…

imitations of casecontrol studies

FInefficient for e+aluation of rare e;posureF 8an=t directly compute ris

F Difficult to esta!lish temporal se4uenceF Determinin" e;posure will often rely on memory

ohort studies


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ohort studies

F u!*ects are selected !y e;posure and followed tosee de+elopment of disease

F wo types of cohort studies

1. ?rospecti+e #classical)

%utcome hasn=t occurred at the !e"innin" of the study It is the commonest and more relia!le

ohort


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ohort…

2. etrospecti+e #Historical)

F Both e;posure and disease has occurred !efore the !e"innin" of the study

F 6aster and more economicalF Data usually incomplete and in accurate

ohort


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ohort…

teps in conductin" cohort studies

1. Define e;posure2. elect e;posed "roup3. elect none;posed "roup'. 6ollow and collect data on outcome

/. 8ompare outcome !7n e;posed - none;posed

ohort


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ohort…

Ad+anta"es of cohort studies

F Ealua!le when e;posure is rareF ;amines multiple effects of a sin"le e;posuresF emporal relationship is nownF Allow direct measurement of ris F @inimize !ias in ascertainment of e;osure


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Eperimental studies


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Eperimental studies

F Indi+iduals are allocated in to treatment andcontrol "roups !y the in+esti"ator

F If properly done, e;perimental studies can produce

hi"h 4uality data

F hey are the "old standard study desi"n

Eperimental


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Eperimental…

;perimental studies can !e1. herapeutic trials

8onducted on patients

o determine the effect of treatment on disease

2. ?re+enti+e trials 8onducted on healthy people o determine the effect of pre+ention on ris


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Eperimental


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Eperimental…

III. Based on o!*ecti+e ?hase I: to determine to;ic effect ?hase II: to determine to;ic effect

?hase III: to determine applica!ility

8hallen"es in inter+ention studies

F thical issues Harmful treatment shouldn=t !e "i+en Gseful treatment shouldn=t !e denied

Eperimental


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Eperimental…

F 6easi!ility issues


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Eperimental…

F he 4uality of U


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Eperimental…

)andomi=ation: random allocation of study su!*ectsin to treatment - control "roups

Ad+anta"e: A+oids !ias - confoundin" Increases confidence on results

$linding: Denyin" information on treatment7controlstatus

Eperimental…


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Eperimental…

Single blinding: study su!*ects don=t now towhich "roup they !elon"

Double blinding: 8are "i+ers also don=t now to

which "roup study su!*ects !elon" #riple blinding: data collectors also don=t now

allocation status

Ad+anta"e: A+oids o!ser+ation !ias

Eperimental…


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Eperimental…

Placebo: an inert material indistin"uisha!le fromacti+e treatment

Placebo effect: tendency to report fa+oura!leresponse re"ardless of physiolo"ical efficacy

?lace!o is used as !lindin" procedure


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?. Screening


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,ims of screening program


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,ims of screening program

F 8han"in" disease pro"ression efficientlyF Alterin" natural course of disease

F ?rotectin" society from conta"ious diseaseF Allocatin" resources rationallyF election of healthy people for *o!

F tudyin" the natural history of disease

riteria for selecting diseases for screening


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e o se ec g d se ses o sc ee g

e+erity he disease should !e serious

reatment arly treatment should !e more !eneficial

?re+alence ?reclinical ?re+alence should !e hi"h

riteria for establishing screening program


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g g p g

he problem should ha+e pu!lic health importance

here should !e accepted treatment for positi+es

Dia"nostic - treatment "acilities should !e a+aila!le

eco"nized latent stage in the time course

3est is accepta!le, relia!le - +alid

7atural history of the disease well understood

/ase4"inding is economical and continuous

Screening tests


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g

he performance of a screenin" test is e+aluateda"ainst a dia"nostic test in 22 ta!le

Diagnostic test

DC D

Screening test C a ! aC!

c d cCdaCc !Cd n

Definitions of cells


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rue positi+es #a): Diseased identified !y test as diseased

6alse positi+es #!): Disease free falsely la!elled as disease

6alse ne"ati+es #c): Diseased falsely la!elled as disease free

rue ne"ati+es #d): Disease free identified as free

Definition of totals


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DC #aCc): total su!*ects with a disease

D

#!Cd): total su!*ects without diseaseC #aC!): total test positi+es

#cCd): total test ne"at+es

#est performance


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p

'alidity of a test

he a!ility of a test to differentiate correctly thosewho ha+e the disease and those who don=t

F It is a function of sensiti+ity and specificity

#est performance


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p

#. Sensitivity o" a test

F he a!ility of a test to correctly identify thosewho ha+e the disease

F he pro!a!ility that a diseased indi+idual willha+e a positi+e test result

F he proportion of people with a disease who

ha+e a positi+e test resultF rue positi+e rate #?)

#est performance


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p

ensiti+ity #?) ?#CWDC)

?7#? C 6> )

@ a"6aAc)

6alse ne"ati+e rate #6>)?# WDC)

c"6aAc7

Sensitivity" #- $%


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#est performance


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p

pecificity #>) ?#WD)

>7#> C 6? )

@ d"6dAb)

6alse positi+e rate #6?)?#C WD)

b"6bAd7

Specificity" #- $P

#est performance


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Predicti!e !alue of a test

he a!ility of a test to predict the presence ora!sence of disease

wo type: ?ositi+e - ne"ati+e predicti+e +alues

#est performance


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#. 5redictive value positive


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#. 5redictive value negative ?#D W ) d"6cAd7

#est performance


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Pre!alence of a diseasehe proportion of indi+iduals with a disease?rior7pretest pro!a!ility of a disease ?re+alence ? #DC) #aCc)7n

Bield of a test

?roportion of cases detected !y the screenin" pro"ram Jield a7n

*ultiple testing


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Parallel testing: ests are "i+en concurrently At least one positi+e indicates disease esults in

F

F Decreased specificity

*ultiple testing


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Serial testing: ests are administered se4uentially All positi+e indicates disease esults in

F ower sensiti+ityF Increased specificity

F Increased ?E?

)eliability of a test


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F A!ility of a test to "i+e consistent results up onrepeated measurements

F wo ma*or factors affect relia!ility @ethod +ariation %!ser+er +ariation

Intero!ser+er +ariation Intrao!ser+er +ariation

)eliability of a test


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elia!ility can !e classified as:

Internal relia!ilityF Internal consistency relia!ility

;ternal relia!ilityF Alternate test relia!ility

F estretest relia!ility

E!aluation of a screening program


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+aluation of a screenin" pro"ram in+ol+esconsideration of two issues

1. -easibility: Determined !y accepta!ility of thescreenin" pro"ram

2. Effecti!eness: Determined !y the outcome of thescreenin" pro"ram

In general


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In "eneral, a screenin" test should !e

elia!le - +alid ensiti+e - specific imple - accepta!le ffecti+e - efficient


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#hank you

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