ResearchMethods: Validity, Reliability, Etc.: Internal and External Validity + Sampling (U6-P2) Sp13

What are internal and external validity?

Is it a valid study? Not: Is it a valid instrument?

Internal validity: Are the conclusions to be trusted for the particular study?





External validity: To whom do the conclusions apply? Generalizability of findings








If something goes wrong in a study, who can you blame it on?  And you can't blame it on the alcohol.  :)


What are some threats to a study’s internal validity?

• Threats due to researcher (e.g., influence results)
• Threats due to how research is conducted (e.g., inaccurate, inconsistent research)
• Threats due to research subjects
• Hawthorne effect
• mortality - loosing people from a study (due to death, etc.)
• maturation - internal change explains behavior

Example: 4 year study of film viewing and levels of prejudice. Subjects= college students.
See any possible threats to internal validity?




What are some threats to a study’s external validity?

• Research procedures don’t reflect everyday life
• ecological validity
• Different finding, same sample
• replication is important
• Poor sampling

Any problems with studies done at universities?
Generalizability?



Sampling

Sampling is the process of selecting subjects for a study.
Why sample?  The population you are studying is too large to study, so you have to study just a part of that population (a sample).

What could be some problems with sampling (examples of poor sampling)?  Bias sample.


You want to use random sampling when you can.  In random sampling all have an equal chance of getting into sample

Types of Samples:

• Simple
• assigned #, random # chosen
• Systematic
• start at random pt., choose every nth subject from list of population
• Stratified
• categories population according a variable studied, and then randomly choose from the categories...see text e.g.!

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