Sampling in Survey Methodology

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Sampling in Survey Methodology

Skerdi Zahaj


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Are Americans romantic?


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A survey conducted specifically to compare findingsto a French survey regarding love and relationships

(Schwartz, 2010).

Are they romantic compared to the French, who arerenowned for their passion for passion?


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(Schwartz, 2010)

In LoveCan true love exist without a radiant

sex life?

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If you want to know what people are doing, observe

them!

If you want to know what people are thinking, askthem!


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Survey

Survey research involves selecting a sample (or samples)

and using a predetermined set of questions.

When a representative sample of people is asked the same

set of questions, we can describe the attitudes of thepopulation from which the sample was drawn.


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If the human population on Earth was homogenous incharacteristics, how many people would an alien need toabduct in order to understand what humans were like?

If all members of a population were identical, thepopulation is considered to be homogenous.

That is, the characteristics of any one individual in thepopulation would be the same as the characteristics of anyother individual (little or no variation among individuals)


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When individual members of a population are differentfrom each other, the population is considered to be

heterogeneous (having significant variation amongindividuals).

How does this change an aliens abduction scheme to find

out more about humans?


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In order to describe a heterogeneous population,

observations of multiple individuals are needed toaccount for all possible characteristics that may

exist.


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Population

Assume youve decided your research question is bestanswered using a survey, and youve determined the

population of interest for your survey.

The next step is to decide who should respond to yoursurvey questions.


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Population

Is any well-defined set of unit of analysis


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Population lists

Countr

yorst

ate

Country or state

Work

ingin

aorgan

Working in a organization

Se

tofco

untr

ie

Set of countries

Cor

poratio

ns

Corporations

Ev

ents

Events

Magaz

i

Magazine articles


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Population

Population = People


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Sample

Any subset ofunits collected in some manner from population

Samplingis the process of selecting a small number of elementsfrom a larger defined target group of elements such that theinformation gathered from the small group will allow judgments to

be made about the larger groups


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The Language of Sampling

Sample frame: a specific list that closely approximates allelements in the populationfrom this the researcher selects

units to create the study sample

Sample frame: The particular population from which asample is actually drown

Sample element: a case or a single unit that is selected froma population and measured in some waythe basis ofanalysis (e.g., an person, thing, specific time, etc.).


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Basics of Sampling Theory

Population

Sampling frame

Sample

Element


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Basics of Sampling Theory

All students on campus

Registrars list of all students

100 students

Student


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What is Sampling?Population

SampleSamplingFrame

Sampling Process

What youwant to talk

about

What youactually

observe inthe data


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Conceptual ModelUniverse

Population

Sample Population

Sample Frame

Elements


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Population or Sample

Populations, not samples, are of primary interest

The power of samples to describe the larger population is

based on the assumption that survey responses in a sample canbe applied to the population from which the sample wasdrawn.


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Why do researchers select a sampleand do not study the population?


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We select the sample

Time Money


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Population or sample

Advantages

Disavantages


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If the sample is selected properly the information may beused to make interference about the whole population.

Sampling arises whenever a researcher takes measurementson a subset of population, however defined, covered by thehypothesis being investigated


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The major threat to representativeness is bias

Biased sample is one in which the distribution ofcharacteristics in the sample is systematically different from

the target population.

A sample of 100 adults that included 80 women and 20 menwould likely be biased if the population were 60% female

and 40% male


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Biased sample

Selection biasprocedures used to selectthe sample result in theoverrepresentation of somesegment of the populationor, conversely, in the

exclusion orunderrepresentation of asignificant segment.

Exit Polls

exit polls are used to surveypeoples attitudes.

demographic characteristics

such as age, race, education,and income of votersinterviewed in exit pollsdiffer from characteristicsof the population.


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Literary Digest poll 1936

Alf. M Landon Franklin D. Roosevelt


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Overrepresented wealthy individuals

The sample frame was selected from :

1. Telephone numbers

2. Automobile ownership


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Types of samples


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We have a problem.

Two student researchers have been asked to do a survey todetermine the attitudes of students toward fraternities and

sororities on campus.

There are 3,200 students in the school. About 25% of thestudents belong to the Greek organizations and 75% do not.

The two student researchers disagree about what samplingplan is best for the study. One researcher thinks they shoulddraw a stratified random sample of200 students: 100 fromamong those students who belong to Greek organizationsand 100 from among the independent students. The secondresearcher thinks they should draw one simple randomsample of 100 students from the campus as a whole.


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The problems

A potentially serious flaw of this plan is that the overallsample would not represent the proportions of Greeks and

independents in the population (25% and 75%,respectively). This would result in a biased sample becauseGreeks would be systematically overrepresented in thesurvey.

The second student researcher is proposing a simplerandom sample of 100 students from the campus

population. While this is likely to lead to a morerepresentative sample, it will probably result in too few

respondents in the Greek category (wed expect about 25Greeks) to adequately represent their viewpoint.


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The solution


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Here it is

A preferred sampling plan would use a stratifiedrandom sample in which the sample sizes for

Greeks and independents are proportional to thepopulation values. With 200 students in the sample,

you would select 150 students from the samplingframe of independent students and 50 students

from the sampling frame of Greek students


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Types of sampling

P

rob

abi

lit

y

Probability sample

Non

pro

b

Nonprobability sample


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Click icon to add picture

Probability sample


Is a sample for which each element in thetotal population has a known probability

of being included in the sample.


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Nonprobability sample


Is a sample for which each element in the totalpopulation has a known probability of beingincluded in the sample.


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Probability sample

Sim

pler

andom

sam

p

l

Simple random sample

Syst

ematic

sam

pl

e

Systematic samples

S

tratif

iedsa

m

Stratified samples

Clu

sters

a

Cluster samples


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Probability sampling is far superior to

nonprobability sampling in ensuring

that selected samples represent the

population.


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Simple random

Each element and combination of elements has an equal chance ofbeing selected.

A list of all elements in the population must be available and a methodof selecting those elements must be used that ensures that each elementhas an equal chance of being selected.


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Random lottery

Write every-day of year on separate slips of paper

Placing the slips in separate capsules

Putting all the capsules in a barrel

After turning the drum, days would be drawn randomly

On December 1, 1969 the dates were drawn one afteranother and given a number

The firs date 14 September was given number 1


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Problems

Anyone with a number 200 or would probably not be called

If birthday was early on, there was a good chance that hewould have serve

Randomness supposedly ensured the systems fairness

Was noticed that people with low numbers tended to beborn in the latter months

The capsules may have been insufficiently mixed


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The steps of Random Samples

Assigni

nganu

mbert

o

Assigning a number to each element in a sample frame

Usi

ngara

ndom

n

Using a random numbertable

Sele

ctasa

m

Select a sample of numbers


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The steps of Random Samples

Po

pulati

on

Population

Dra

w

Draw

Star

t

Start


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By the lot method

Elements are drawn outrandomly until the

desired sample size hasbeen reached.

Click to edit Master text stylesSecond level

Third level Fourth level

Fifth level


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In Simple RandomSampling

The gap, or period between successive elements is

random, uneven, has no particular pattern


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Simple random sampling requires a

list of the members of population

The conclusion


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Systematic Sample


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Systematic Sample

Elements are selectedfrom a list at

predetermined intervals

Elements are chosen

from the listsystematically ratherthan randomly

Everyxth element onthe list is selected

This number is calledsampling interval


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In SystematicSamplingGaps between elements are equal andConstant There is periodicity.


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Problem????

We want to draw a sample of 100 names from a list of5000 students attending a college.

We would go through the list andselect every fiftieth students

We would select a random start


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Systematic sampling

We deal with a long listof population elements


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Disadvantages

Elements of the listhave been ranked

according to acharacteristic

Students grades

People income

.


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Disadvantages

List contains a patternthat corresponds to

the sampling interval

Attitudes of children

from large families

Sampling interval is 5

Families included in thelist have 5 children

We will sample all thechildren who were in

the same position

Birth Order


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Stratified sample

Probability sample inwhich elements sharing

one or morecharacteristics aregrouped

Divide the population by certaincharacteristics into homogeneoussubgroups (strata) (e.g., UI PhD

students, Masters Students,Bachelors students).

Elements within each strataare homogeneous, but areheterogeneous across strata.


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A simple random or a systematic sample is taken from each stratarelative to the proportion of that stratum to each of the others.

Stratified sample

Proportionate (Each stratum is representedin proportion to its size in the population)


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POPULATION

STRATA 1STRATA2

n = 1000; SE = 10%

n= 500; SE=7.5% n = 500; SE=7.5%

Equal intensity


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POPULATION

STRATA 1

STRATA 2

n =1000, SE = 10%

n = 600SE=5.0%

n =400SE=7.5%

Proportional to size


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Sample equal intensity vs.?proportional to size ?

What do you want to do? Describe the population,ordescribe each strata?

1. Equal intensity

2. Proportional size


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Stratified sample

Characteristics that are expected to be related to or affectthe depended variable in the study.

Measure the average income of households

Education Sex Race Household head


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Conducting a survey of 200

500

liberal

artmajo

r

500 liberal art major

100enginee

ringmaj

o

100 engineering major

200busi

nessmaj

200 business major

125li

beral

artma

jo

r


25eng

ineeri

ngma

jo

r


50

busin

essma

jo

50 business major


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If we want to investigate some questions in eachmajor separately

Number LA Engin Business Total

Stud. 500 100 200 800

P.W 0.625 0.125 0.25 1

Size

Sample

100 50 50 200

Samplemeangrade

2.5 3.3 2.7 2.65

100li

beral

artma

jo

r


50eng

ineeri

ngma

jo

r


50

busin

essma

jo

50 business major

Weighting factor


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Describe thepopulation

The mean would be = 0.625 (2.5) + 0.125 (3.3) + 0.25 (2.7)= 2.65


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Cluster SamplesThe list of elements does not exist


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Cluster sample

Is a probability sample in which the sampling frameinitially consist of cluster of element

It is groups or clusters of elements are identified and listedas sampling unit

The sample is drown from the list of sampling units


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POPULATION

CLUMP


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POPULATION

Primary samplingUnit

POPULATION


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POPULATION

= Randomly selected PRIMARY SAMPLING UNITS.


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Cluster samplingSome populations are spread out (over a state or country).

Elements occur in clumps (towns, districts)Primary samplingunits.

Elements are hard to reach and identify.

You cannot assume that any one clump is better or worse thananother clump.


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Opinion Poll of 1000 people in a city

Obt

ainingama

pofth

e

Obtaining a map of thecity

Identi

fyingandli

s

Identifying and listing allblocks

Sm

allnum

berof

b

Small number of blocks are sampled at random orsystematically

Sele

ctthe

d

Select the dwellingunits

Contac

t

Conta

ct


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Opinion Poll of 1000 people in a city

1

2

3

The probability of one household being selected is :25/500 x 1000/4000 = 1/80


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Cluster sampling

Reduces fieldwork cost

Reduces travel andadministrative costs


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Errors

Errors occur at each stage of the cluster sample

The random errors at each levels must be added together to arrive at

the total sampling error for a cluster sample


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What to chose?

40 individuals from 25 blocks

25 individuals from 40 blocks

Homogenous population

States regions


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Nonprobability Samples


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NonprobabilitySamples

Is a sample for whicheach element in the total

population has aunknown probability ofbeing selected



Fifth level

P


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Pur

posiv

esam

ple

Purposive sample

Conve

nienc

esam

pl

Convenience sample

Qu

otasa

mple

Quota sample

Snow

ballsa

m

Snowball sample


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To expensive

Unusual cases


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Purposive sample

Researcher exercisesconsiderable discretion

over what observationto study

Study a diverse andusually limited numberof observations


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Convenience Sample

Elements are includedbecause they are

convenient or easy toreachI havesomequestions


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Quota sample

Elements are sampledin proportion to their

representation in thepopulation

Stratified vs Quota ?

Elements in the quota

sample are not chosenin a probabilisticmanner


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Quota sample

550 women

450 men

Select whomever theypleased until the

quota were reached


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Snowball sample

The respondents areused to identify other

persons who mightqualify for inclusion inthe sample.

The elements are askedto supply appropriatenames for furtherinterviewing

We interview untilenough persons are

interviewed to satisf


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Snowball sample


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Sample Statistics

Parameter: any characteristic of a population that is trueknown on the basis of a census (e.g., % of males orfemales; % of college students in a population).

Estimate: any characteristic of a sample that is estimatedestimated on the basis of samples (e.g., % of males or

females; % of college students in a sample)


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Sampling error

An estimate of precision; estimates how close sampleestimates are to a true population value for a characteristic.

Occurs as a result of selecting a sample rather thansurveying an entire population

A measure of


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S t a n d a r d E r r o r

A measure ofsampling error.

Sample size As sample size


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SE is an inverse function of sample size

Sample size

Standard

pincreases, SEdecreasesthesample is more

precise.

So, we want to usethe smallest SE wecan greatest

precision!

When in doubtincrease sample size.



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How large a sample?


Fifth level


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?


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Sampling in Survey Methodology

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