Cohort StudiesDr. Sameh Zaytoun

(MBBch, DPH, DM, FRCP(Manch), DTM&H(UK) ,Dr.PH)University of Alexandria - Egypt

Consultant of Preventive MedicineAl- Hada Armed Forces Hospital

Cohort

Term taken from the ancient Roman term for a group of soldiers that marched together into a battle field.

i.e (a group of subjects followed together over time).

Cohort study DesignDesign in Words:Select representative persons with the study exposureSelect comparable Controls without the study

exposureFollow both groups equally to assess who develops

the outcomeCompare Risk of developing the outcome in the

Exposed and in their Controls

Interpretation of Cohort study

All else being equal, if the Risk of the Outcome differs between the Exposed and the Non-Exposed by more than chance variation, the Exposure is said to be related to the Outcome

Analysis of Ideal Cohort Study Design

Exposure Present

Exposure Absent

Outcome Present

Outcome Absent

Outcome Present

Outcome Absent

Outcome

+ ve

Outcome

- veTotal Risk Odds

Exposure + ve A B A + B A / (A+B) A / B

Exposure - ve C D C + D C / (C +D) C / D

Risk Ratio =A / (A+B)

C / (C+D)Strength of Association

Odds Ratio =A / B

C / D= AD / BC

Timing of Cohort study

Concurrent Cohort Study: The cohort is assembled in the present and followed into the future to determine disease incidence

Retrospective Cohort Study: The cohort was assembled in the past and determination of disease incidence is conducted in the present

Concurrent Cohort study

Advantages:Temporal relation between Exposure and

Disease can be established reliablyExposure can be measured accurately not

dependent on recall or recordsExposure assessment not Biased by

knowledge of the Outcome

Concurrent Cohort study

Disadvantages:

Expensive and time consuming Can not be used for Rare Diseases

Retrospective Cohort study

Advantages:Same Advantages as concurrent cohort

studiesMuch less costly and time - consuming

Retrospective Cohort study

Disadvantages:Can only be used if data are available

about Exposure, Disease, and Confounders at start of the follow up period

Investigator has no control over quality of Exposure measurements

Selection of Subjects:

Exclude subjects who are not capable of developing the disease (e.g. women with hysterectomy in a study on uterine carcinoma)

Restrict to subgroups with a fairly high incidence of the disease (e.g hip fracture in the elderly)

Obtaining Exposure information: Quality of results depends on quality of

measurements Some Exposure variables may change over

the follow – up period (e.g. exercise habits), so multiple measurements may be required

Obtaining Disease information:

Outcomes should be assessed using a standardized criteria

Outcomes should be assessed blindly without knowledge of exposure status

Reducing loss to follow – up: Exclude subjects who will be very difficult to

follow Collect information (e.g. name of subject

physician, relative) that will allow subjects to be found if they move or die

Maintain periodic contact during the follow – up period (e.g yearly phone calls, birthday cards)

Difficult -to- Trace Subjects:Contact the subject physician, relativesRequest forwarding address from the postal

servicedetermine vital status from the National Death

Index (in the United Kingdom)seek address through other sources (e.g. life

insurance, voter registration, police offices…..)

Person - Time Concept: Used to account for person –to- person differences in time

under observation Each person is given credit for just (only) the time being

observed Person .Years = Time in years x persons followed Time and Persons followed are weighted equally - 10 persons followed 1 year = 1 person followed 10

years = 10 person.years - best when risk of outcome is relatively homogeneous

over time and degree of exposure is equal over time.

Cohort points of Vulnerability:Ascertainment of Study Outcome without

Bias: Assessment Bias: - Due to knowledge of Exposure - Increase the publicity about the possible association Remedies: - Mask assessors from the Exposure status - Use standard protocol to assess / validate the Outcome - Assess more than one Outcome to avoid telegraphing

the exact association sought

Comparison:Case -Control

Concurrent Cohort

Retrospective cohort

Study time shortest long Short

Cost Low Highest High

Rare Disease Yes No No

Recall Bias Yes No No

Sample Size Small Large Large

Loss to follow up No Yes Yes

Incidence No Yes Yes

Relative Risk Approx. Yes Yes

Odds ratio Yes Yes Yes