Answer to Question 1
External validity is the extent to which the results of an experiment can be generalized. Because most research in psychology is completed on college sophomores, generalizability (or external validity) can be problematic.
Answer to Question 2
In a proportionate stratified sample, each of the subgroups in the sample has exactly the same proportion as the corresponding subgroup in the population. For example, if 14 of the population is between 30 and 39 years old, then exactly 14 of the sample will be in this same age range. Thus, the composition of the sample exactly matches the composition of the population. A stratified random sample has equal representation from each of the specified subgroups. For example, if there are four subgroups in the population, then each will form 25 of the sample. However, the subgroups are probably not equally represented in the population. For example, one group may comprise only 10 of the population but will still make up 25 of the sample. Thus, the composition of the sample does not accurately represent the composition of the population. Finally, simple random sampling lets pure chance determine who is selected. As a result, a group that makes up 10 of the population can be severely overrepresented or underrepresented in the sample, depending entirely on chance. Thus, there is no guarantee that the composition of the sample will accurately represent the composition of the population.
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