The document discusses two types of validity: internal validity and external validity. Internal validity refers to the extent to which a study's findings reflect the true causal effect in the target population. External validity refers to the extent to which findings from a study population can be generalized to an external population beyond the target population. A key point is that internal validity is a prerequisite for external validity - a study must first be internally valid before its findings can be generalized to other populations.

1. 1
Types of validity
• Overall definition of validity: A study design
and analysis plan are valid if, on average
(aside from random error), results generated
from the study design reflect the true causal
effect in the source population*
• A faulty study design and/or analysis plan will,
on average, produce biased results...meaning
that results will not reflect the true causal effect
in the source population*

2. Populations and types of validity
Study population
(sample)
Actual Population
Target Population
External Population
Statistical
inference
Internal
validity
External
validity
JC: “Sample”
JC: ”Available”
JC: “RQ”
JC: “Generalizable”
33
Adapted from Kleinbaum, Kupper and Morgenstern, 1982

3. 3
Types of validity
• There are two types of validity depending on
the source population of interest
– Internal validity
– External validity
• Can understand these types of validity in the
context of the hierarchy of populations

4. 4
Types of validity
• * Type of validity depends on the source
population of interest for generalization
• Internal validity:
– The extent to which findings from the study
population reflect the true underlying
parameters/causal effect in the target population
• External validity:
– The extent to which findings from the study
population reflect the true underlying
parameters/causal effect in a defined external
population beyond the target population

5. Populations and types of validity
Study population
(sample)
Actual Population
Target Population
External Population
Statistical
inference
Internal
validity
External
validity
JC: “Sample”
JC: ”Available”
JC: “RQ”
JC: “Generalizable”
36
Adapted from Kleinbaum, Kupper and Morgenstern, 1982

6. 6
Types of validity
• Internal validity is a pre-requisite for external
validity, and is the primary concern
• A study may lack external validity, but even if
we can’t generalize results to some external
population, the findings are still meaningful if
the study is internally valid
• If a study lacks internal validity then it
necessarily lacks external validity as well
• As such, lack of internal validity is the primary
concern