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External Validity
- Types of Research Validity
- Components of External Validity
- Population
- Setting
- Task/Stimulus
- Participant Selection -- Population Validity
- Internal
- Statistical conclusion
Bivariate RH:s, Research Designs and Validity...
A RH: is a guess about the relationships between behaviors
In order to test our RH: we have to decide on a research design, sample participants, collect data, statistically analyze those data and make a final conclusion about whether or not our results support our RH:
When we are all done, we want our conclusion to be “ valid”
Validity … has lots of types, definitions & procedures
but basically it means … Accuracy or Correctness
Important to remember !!! No one study, no matter how well-
done can ever be conclusive !! You must further apply the
research loop -- replication and convergence are necessary
before you can be sure about the final answer to your RH:
Types of Validity
Measurement Validity
- do our variables/data accurately represent the behaviors we intend to study?
External Validity
- to what extent can our results can be accurately generalized to other participants, situations, and times?
Internal Validity
- is it correct to give a causal interpretation to the relationship we found between the variables/behaviors?
Statistical Conclusion Validity
- have we reached the correct conclusion about whether or not there is a relationship between the variables/behaviors we are studying?
How types of validity interrelate -- consider the “flow” of a study
the research “design” -- all the choices of how we will run the study
Internal validity
- control
- causal interpretability
External validity
- generalizability
- applicability
the data -- if we can’t get an accurate measure of a behavior we can’t study that behavior
Measurement Validity
the data analysis -- we must decide whether or not the behaviors we are studying are related (and if so, how)
Statistical Conclusion Validity
Components of External Validity
Whether we are testing attributive, associative, or causal research hypotheses, we should be concerned about the generalizability of the research results
Population
- Will the results generalize to other persons or animals?
- Will a study of college students generalize to your target population of “consumers”?
- Will a study of chronically depressed patients transfer to a those who are acutely depressed?
- Will a study of captive bred turtles generalize to wild- caught turtles?
Setting
- Will the findings apply to other settings?
- Will a laboratory study generalize to what happens in the classroom?
- Will a study in a psychiatric hospital generalize to a out- patient clinic?
- Will a laboratory study generalize to retail stores?
Components of External Validity, cont.
Task/Stimuli
- Will the results generalize to other tasks or stimuli?
- Usually the participant is “doing something” that directly or indirectly generates the behavior that is being measured - Will a “lever pressing” task tell us anything about “compliment seeking”? - What do I learn about “consumer decision making” from a study that asks participants to select the best “wigit”? - Will research using visual illusions inform us about the perception of everyday objects?
Societal/Temporal changes
- Will the findings continue to apply
- Will a study conducted in 1965 generalize to today?
- Will a study conducted today still be useful 10 years from now? … 5 years from now?
Here are two other related types of validity -- both of which can be understood as specific combination of certain elements of external validity...
Cultural Validity -- different behaviors or relationships between behaviors across cultures
- a culture is jointly defined by its members and location
- this can be expressed as a combination of population and setting components of external validity
Ecological Validity -- original discussions of this involved whether or not the study “engaged” the participants and produced “realistic behaviors” (e.g., mock juries deliberation vs. individual paper and pencil responses)
- the “ecology” of a study includes the elements that the participant interacts with and within
- this can be expressed as a combination of setting and task/stimulus components of external validity
So, external validity is about the “generalizability” or “applicability” of the results of a study.
- It’s important to distinguish generalizability and applicability! Generalizability of a finding (broadly speaking) is whether or not the results will hold for all combinations of the elements of external validity. Applicability of a finding is whether or not the results will hold for a particular combination of the elements of external validity for which we’d like to use the results So, generalizability is much more demanding than applicability, especially in terms of the convergent research required to support claims of each. Also, generalizability is more difficult to evaluate because it requires a deeper knowledge of the extent to which population, setting and task/stimulus differences will influence research findings – the more psychology you know the better…
Generalizability is a property of the study (and is
certainly always limited!), but applicability is in the eye
of the applier”
Approaches to “defending” limited external validity of a study…
De-emphasize external validity (emphasize Internal Validity)
- if the main focus of the study is causal interpretability (internal validity), you might make a large number of choices each of which hinders the generalizability of the results
- common among theoretical researchers -- but doesn’t help the applications folks (& why we have “applied research”)
Eschew external validity (emphasize focused applicability)
- basically the argument is that this study used exactly the pop, setting, task, stimulus, etc. that was of interest to the researcher
- common among applied researchers •“my researcher exactly matches my application; what’s to generalize?”
- “my researcher exactly matches my application; generalization to your application is your problem!”
Participant Selection / Sampling”
¾ “Who will be in the study?”
¾ goal is to have a sample that represents the target population
¾ related type of validity is External Validity -- Population
¾ Note -- participant selection (sampling) has nothing to do with the causal interpretability (internal validity) of the study results -- only the “Population” component of External Validity !!!!!
Stages of Selection/Sampling
Target Population – defining people/animals we want to study
Sampling Frame – “best list” we can get of population members
Selected Sample – sampling frame members who are selected to participate in the research
Data Sample – participants from whom useful data are collected
Identify each -- telling the number, if possible…
For our study of UNL students we collected complete data from 72 of the 100 students that were selected from a data file of all UNL undergraduates
population selected sample
sampling frame data sample
For our study of California voters, we obtained the names of all registered voters in that state, selected 2000 and collected data from 1214.
population selected sample
sampling frame data sample
Comments on sampling ?? Poor purposive sampling frame used ... “UNL students” vs. “UNL undergrads”
Selection/Sampling Procedures
Psychologists have devised many different ways of “acquiring” participants, but all involve three choices…
- Population Sampling Frame vs. Purposive Sampling Frame
- Researcher selected vs. Group invitation/Self-selected
- Simple Sampling vs. Stratified Sampling
… any form of participant sampling/selection can be identified as one of the (eight) combinations of these three choices
In an important sense -- all participants are “volunteers”
- participants must be invited with full knowledge of any risks incurred through their participation
- they might refuse to participate when invited
- they might start to participate but later withdraw -- called attrition, drop-out or “experimental mortality”
Researcher selected vs. Group invitation/Self-selected, cont.
Group invitation/Self-selection -- all potential participants
from the sampling frame are informed about the “opportunity” to
participate in the research and invited to contact the researcher if
they wish to volunteer.
- Assumes that the volunteers will be a “representative sample” of the target population
- This representativeness can be compromised if ...
- the entire target population is not notified
- if there is “uneven” motivation to volunteer across the population (e.g., a small payment for participation would lead to differential representation of those who do and don’t find that amount “motivating”)
Some practice… identify “Researcher-selected” vs “Self-selected”
- 40 folks are selected from the Lancaster County voter registration rolls and each contacted to participate
- Research announcements & invitations are mailed to all 12,234 on the Tali County voter registration rolls
- Psyc 181 research participation website was used to recruit 100 participants
- Harris labs selected 30 folks who had previously been research participants and who had indicated their interest in further participation to be part of their latest study
- Using the Psyc 181 grade roster, 200 research participants were selected.
- Advertisement for Harris Labs research that requires non-smokers aged 21-39 printed in local newspaper
“Kinds” of Selection/Sampling, cont.
Simple Sampling vs. Stratified Sampling
In “simple” sampling every member of the sampling frame
has an equal probability of being in the study
- every name on list has the same probability of being chosen
- every volunteer participant completes the study
“ Stratified ” sampling is a bit more involved …
- first we have to divide the sampling frame into “strata” using one or more variables (e.g., age, gender, job)
- members within each strata have an equal probability of being in the study
- usually done to ensure representation of smaller segments or strata of the population
- select 50 each of “Psyc majors” and “non-majors” from 181 rosters
- have separate sign-up sheets for “majors” and “non-majors”
Some Practice -- is each an example of “simple” or “stratified” sampling ???
- We chose 40 African-Americans, 40 Asian-Americans, 40 European-Americans, and 40 Hispanic-Americans from the rolls of the Multicultural Club
- We chose 100 folks from the Registrar’s student list
- Our participants were the first 40 folks who responded to the research participation advertisement
•After we’d had 50 male and 35 female volunteers, we changed the sign-up sheet to read “females only”
- (Careful!) Our intention was that the 200 students selected from the Psyc 181 course roster would be 70% from the College of Arts & Sciences and 30% from other colleges.
- We sorted the Psyc 181 course roster into those from the College of Arts & Sciences vs. other colleges; then we chose 70 of the former and 30 of the latter
So, there are 8 combinations of ways we obtain our participants...
Simple Stratified sampling sampling
Simple Stratified sampling sampling
Researcher- selected
Self-selected
Population sampling Purposive sampling frame frame
* what “random sampling” means in textbooks
^ how “random sampling” is usually done (e.g., Gallup polls)
^ ^
- how “participant selection” is usually done in empirical research
Time for practice… identify each as 1) complete or purposive sampling frame ...
- researcher- or self-selected ...
- simple or stratified … sampling
We chose 40 African-Americans, 40 Asian-Americans, 40 European-Americans, and 40 Hispanic-Americans from the rolls of the Multicultural Club to gather information about opinions of members of the Multicultural Club.
We chose 160 members from the rolls of the Multicultural Club to gather information about opinions of students at UNL.
We posted two notices on the Multicultural Club bulletin board about a “forum” we were hosting to gather information about the opinions of college students, one for men and one for women.
