Agenda Sampling

probability sampling

nonprobability sampling

External validity

SamplingDrawing a subgroup from a population

(vs. Census)

Population Sample

Students registered in FAMR 380 fall ’00

All registered HI voters in Sept. 2000

All Adidas shoes made in 1999

Everyone in class today

Registered voters reached by random-digit dialing on 9/14/00 who answered the survey

Every 2,000th pair produced at each plant

What is a good sample?A sample that resembles the population

in characteristics (a representative sample)

RepresentativenessPopulationSample

RepresentativenessPopulationSample

40% Males

60% Females

40% Males

60% Females

RepresentativenessPopulationSample

70% Satisfied

30% Dissatisfied

70% Satisfied

30% Dissatisfied

Statistics Parameter

Why Representative Sample? If characteristics of the sample is similar

to the population, the statistics of sample are likely to be similar to the parameters

Let’s Think …Research question: How do UH

students utilize campus facilities?Population: UH StudentsSample size: 200How will you sample? How can you maximize

representativeness of your sample?

Random SamplingGives everybody in the population an

equal chance to be selected as a participant in the sample

Requires the list of everybody in the population

SIMPLE RANDOM SAMPLE

Population of 40:25% 25%50%

Sample of 4 :Each person 1/10 chance

Sample A

Sample B

Sample CSample D

Systematic Random SamplingPick up every ‘n’th subjectsSensitive to the way the list is ordered

SYSTEMATIC SAMPLE

Population of 40:25% 25%50%

For a sample of 4,Take every 10th one

Sample B

Sample A

Stratified Random SamplingDivide the population into groups (strata)Select subject randomly from the stratumThen proportion of groups in the sample is equal to

proportion of groups in population

STRATIFIED RANDOM SAMPLE

Population of 40:25%25%50%

Stratify (layer, category)by color Stratified random sample of 4:

Randomly pick from each strata to maintain 25%, 25%, 50% balance

Cluster SamplingSample a ready-made group within the

population (cluster) assuming it has a similar composition to the population

Example: Third grade classrooms

• Know exact chance of being included BEFORE participant is picked• E.g., 1 in 100, .003%, etc.• Need # in the population, • # in sample

• DON’T know each participant’s chance of being picked

Probability vs. Non-probability

Simple random Systematic random Stratified random Cluster Each member of the population has

a specifiable probability of being chosen

Population info available

Convenience Snowball Purposive Quota We don’t know the

probability of a specific member of the population being chosen

Population info not available

Probability Sampling

Non-probability Sampling

Representativenss & Generalizability

Representativeness = Resembles population characteristics

Generalizability = Able to generalize the results of your study to the whole population

High representativeness = High generalizability

Probability sampling allows higher representativeness than non-probability

Non-probability Sampling

Convenience SamplingGet available people in the

populationLow representativeness /

generalizability

Snowball SamplingObtain participants through a chain

of personal networking-referralsUseful to locate the ‘hidden’ or

‘difficult to recruit’ populationLow representativeness /

generalizability

Quota SamplingPredetermine the proportion of

groups in the sample e.g., male 50%, female 50% e.g., clinical trials-drug research, etc.

Purpose of Quota Sampling 1: To ensure that the sample reflects the

proportion of the group in the population

2: To secure enough numbers of group members for analysis

If you set quota for purpose 2, your sample may not reflect the population as a whole

Purposive Sampling

Obtain most informed / most ‘typical’ participants

“Judgmental sampling”High quality of information from each

participanta Low representativeness /

generalizability Quality of sample depends on

researcher’s ability to identify group to be studied

Why is sampling important?

Usually want to talk about a POPULATION

Easier to get a sub-set of the population (SAMPLE)

In a good sample Results from a good sample should match the

parent population (REPRESENTATIVE) Participants should be chosen without bias

(RANDOM)

This allows you to GENERALIZE the results-- what holds for the sample should also hold for the larger group

External ValidityDegree that results can be extended

beyond the limited research settingExtent findings can be generalized to

others

Based on sample ( rats, college students, whites, males, lab setting)

External validity ?Will the findings from this study likely be

found When other individuals are studied?

Volunteers / non-volunteers, Gender

Under other conditions? In other settings?

PsychologyThe study of college sophomoresPeople in general?College students - intelligent, high

cognitive skills, young, developing sense of self-identity, social and political attitudes in state of flux, need for peer approval, unstable peer relationships (Cozby, 2001).

External validityRelated to sample and sampling

technique

I hope you have a

great day!