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