Exam #2 (2 of 2) Flashcards
SAMPLING AND GENERALIZATION
SAMPLING AND GENERALIZATION
Population and Census
Population = the entire set of people the reseacher desires to learn about
Census = measures each person in the population (not feasible for the research we do)
Sampling and Sample
Sampling = the selection of certain individuals to participate in the research
* The researcher uses information about these individuals to make inferences about the population from which they were drawn
* relates to inclusion/exclusion criteria, etc.
Sample = the people who actually participate in the research
* Researchers will not be able to know exactly the true characteristics of a population, but a representative sample helps facilitates these inferences
* representative samples are approximately the same as the population in every important respect
Probability sampling
Probability sampling is used to draw a representative sample from the population of interest
* –> each person in the population has a known and non-zero (though not necessarily equal) chance of being selected
* probability sampling often used in survey research/political polling
Types of probability sampling
Types of probability sampling:
- Simple random sampling
- Systematic random sampling
- Stratified sampling
- Disproportionate stratified sampling
- Cluster sampling
Simple random sampling
Simple random sampling:
* Participants are randomly selected from a list of all members of the population of interest
* Every person on the list has the same chance of being selected
* Sampling frame = complete list of all the people in the population
Systematic random sampling
Systematic random sampling
- With systematic random sampling, the names on the sampling frame are known to be listed in random sequence
- A random starting point can be chosen
- Ever nth name can be selected
- Example: 5 is the random starting point chosen, then every 5th person chosen until we get 20 people
Stratified sampling
Stratified sampling
- Stratified sampling is used when we want to make sure that some characteristic is represented in the sample in the same proportion as it is represented in the population
- Involves drawing separate samples from a set of known subgroups called strata rather than sampling from the population as a whole
- Example: Two strata of IUP graduate and undergraduate students
Disproportionate stratified sampling
Disproportionate stratified sampling
* Disproportionate stratified sampling is used when the strata differ in size and we want to include enough people from each one to allow us to compare the characteristics of the strata
* This approach involves a technique called oversampling = drawing a sample that includes a larger proportion of some strata than they are represented in the population
* Example: Inviting all students from a small department and a small # of students from a large department to attempt getting equal groups in each stratum
Cluster sampling
Cluster sampling
* Cluster sampling can be used when a complete sampling frame does not exist
* We break the population into a set of smaller groups, called clusters
* Then we randomly choose some of the clusters until we reach a level where there are sampling frames
Example: study about alcohol use and the way students spend their time on college campuses. Not possible to get students from every university. Instead, draw from 5 random states. Then draw from 5 universities from list of state’s universities. Then conduct research on 25% of students
* –> go through a randomized process to get sampling frames
Representative sampling & sampling bias
Representative sampling requires:
* The existence of one or more sampling frames listing the entire population of interest, AND
* All selected individuals must be sampled
Sampling bias occurs when either of these conditions is not met
* There is the potential that the sample is not representative of the population
Nonprobability sampling
Nonprobability sampling
- When probability sampling is not possible, nonprobability samples must be used
- Examples of nonprobability sampling include:
- –Snowball sampling
- –Convenience sampling
Snowball sampling
Snowball sampling is used when members of the population of interest are rare or difficult to reach
* One or more individuals from the population are contacted
* These individuals lead the researcher to other members of the population
* Example: New parents sharing study with other new parents they know
Convenience sampling
Convenience sampling
* When convenience sampling is used, the researcher samples individuals who are readily available without any attempt to draw a sample that is representative of a certain population
* Even though psych 101 students are all psych students, the researchers aren’t studying them for that specific characteristic. Hence, convenience sampling
Random sampling vs. Random assignment
Random sampling selects participants for the research on the basis of chance
* Random sampling strengthens EXTERNAL validity by drawing a representative sample
Random assignment places people into experimental conditions on the basis of chance
* Random assignment strengthens INTERNAL validity by evenly distributing participant characteristics across conditions and thus ruling out selection threat
GENERALIZED CAUSAL INFERENCE
GENERALIZED CAUSAL INFERENCE
Relationships between generalizability and construct validity/external validity
Construct validity: validity of inferences about generalizing from specific operationalizations to higher order abstract constructs
External validity: validity of inferences about generalizing the cause-effect relationship to other persons, settings, treatments, and measurements
Mnemonic for remembering the 5 principles of generalized causal inference
5 principles of generalized causal inference mnemonic:
Snakes rarely meet in caves
5 Principles of Generalized Causal Inference
5 Principles of Generalized Causal Inference:
- Surface similarity
- Ruling out irrelevancies
- Making discriminations
- Interpolation and extrapolation
- Causal explanation
- Surface similarity
- Surface similarity
- How similar do the particulars of the study seem to be to the prototypical features of the target of generalization?
- Surface similarity:
Applications to construct validity
- Face validity: do the study particulars appear to capture the intended constructs?
- Content validity: do the study particulars capture all the prototypical features of the intended constructs?
- Inadequate explication of constructs impedes these judgments
- Surface similarity:
* Applications to external validity
- We often make judgments about likely generalizability of research findings based on how similar the study particulars are to the target of generalization (will the intervention work well in both Illinois and Rhode Island? Consider similarities of settings)
- Campbell’s principle of proximal similarity = greater similarity leads to greater confidence about generalization
2. Ruling out irrelevancies
2. Ruling out irrelevancies
- What characteristics of the people, settings, treatments, and outcomes are irrelevant (because they do not limit generalization)?
- They do not qualify the effect that we are finding –> don’t limit generalizability
2. Ruling out irrelevancies
Applications to construct validity
Multiple operationalism: Across the operationalizations of a construct, there should be heterogeneity of irrelevant features. This rules out the mono-method and mono-operation threats to construct validity
- Example: Social psych studies. Some involve computers, some ink blots, etc., These features are irrelevant as long as they are measuring the same construct
Campbell: heterogeneity of irrelevancies help make sure the construct is not confounded even with apparently irrelevant features - Think about within context of mono-method and mono-operation: are we really getting at this bigger construct we’re interested in?