External Validity
Process:
step 1: define a target population of individuals, settings, or times
step 2: draw samples from those populations
two types:
representative sample: samples that correspond to a well known population
very rare
accidental samples or samples of convenience: achieved by a procedure designed to ensure representativeness
may or may not be representative but you might not know
Features of external validity
1. generalizing to particular target individuals , settings, and times
e.g., does this information hold true for other individuals under the same circumstance
2. generalizing across types of individuals, settings, and times
e.g., does this information hold true for different individuals under different circumstance
Threats to External Validity
1. Interactin of selection and treatment
population is adequately targeted, but the results are only applicable to the participants who show up.
(volunteers, exhibitionists, hypochondriacs, scientific dogooders)
Solution: Make participation as convenient as possible and attractive to get overall participation
2. Interaction of setting and treatment
this is where the effect of treatment is dependent on the setting
e.g., can you get the same results at a factory as you do at a university
Solution: Vary setting and see if the same patterns show up
3. Interaction of History and treatment
this is when different times results in different results
an example is the cohort effect
e.g., national tragedy
Solution: replication