4. External Validity
This aspect of validity relates to the degree that research findings (from a sample) can be
generalized or extended to (1) the population or (2) similar contexts. Aspects of the sample are
critical when evaluating external validity. Recall that aspects of the research design and in
particular the research method are critical when evaluating internal validity.
We will review THREE sampling approaches that affect external validity.
a) Probability Sampling
In this approach, you must begin by identifying a population. After you have (1) defined a
population, (2) you draw a sample of participants from the population, then (3) conduct your
study to reveal findings that pertain to the random sample or participants, and then (4) you
generalize the findings you obtained from the random sample back to the population.
You can mathematically calculate the probability that your sample is representative of your
For example, 500 objects or people are defined in your population. You randomly select 100. So
the probability that your sample is representative of the population is 20%. The higher the
percent; the greater the external validity. This is why sample size affects external validity.
b) Random Sampling
If you randomly select a sample from a defined population, this increases external validity as
well. Random sampling means that each object or individual in the population had an equal
chance of being selected as a participant in the study.
c) Purposeful Sampling
This type of sampling occurs when it is not possible to randomly select participants from the
population nor calculate the number of object