Agenda Sampling probability sampling nonprobability sampling External validity.
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Transcript of Agenda Sampling probability sampling nonprobability sampling External validity.
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!