Chapter 7: external validity

population

the entire set of people or products you are interested in

sample

a smaller set taken from the larger population

census

a collection of data from an entire population

probability sampling

All members of the population have an equal chance of being included in the sample:

• unbiased sample

• representative sample

• random sample

• probability sample

We have external validity

non-probability sampling

Some members of the population have a higher probability of being included in the sample than others:

• biased sample

• unrepresentative sample

• nonrandom sample

• nonprobability sample

We have unknown external validity

convenience sampling

sampling only those who are easy to contact

self-selection

sampling only those who volunteer

simple random sampling

the most basic form of probability sampling

— assign everyone in a population a number and use a random process to select those numbers (like a lottery)

systematic sampling

generate two random numbers and use those to systematically select individuals from the population (e.g., start with the 8th person and select every 33rd person after that)

cluster sampling

randomly select subgroups, and sample everyone within those groups

– subgroups are arbitrary

stratified random sampling

select particular demographic categories and then

randomly select people within those categories to keep the numbers proportionate to the population

– subgroups are meaningful

oversampling

sampling more of a particular demographic category to increase validity of statistical estimates

– subgroups are meaningful

convenience sampling

using a sample of people who are easy to contact and readily available to participate

– psychology professors recruiting psych

students to participate in studies

– collecting data on online platforms

purposive sampling

when you want to study certain kinds of people, you purposefully recruit only those kinds of participants

– recruiting smokers using flyers in a tobacco store

snowball sampling

participants are asked to recommend other participants for the study

– asking smokers to recruit other people in their support group, who then recruit more, etc.

– useful for sampling rare populations

quota sampling

identify subsets of the population, set a target number (i.e., quota) for each category, and use nonrandom sampling methods to reach the quotas

– similar to stratified random sampling in that there are meaningful subcategories (e.g., demographics)

– but participants in each category are selected using e.g., convenience sampling, snowball sampling

Bystander effect

in a 1968 study, people were less likely to help an
anonymous victim when there were bystanders present