A brief overview of threats to internal validity. A quick overview or a recap, before or after (respectively) going into in-depth information.

1. THREATS TO INTERNAL VALIDITY
Confounding and
Extraneous variables
Environmental
Variables
Time-related
Variables
Individual
differences

2. A valid research study would have to demonstrate that changes
in one variable (for example, the amount of exercise) are
followed by changes in the other variable (cholesterol level),
and that no other variable provides an alternative explanation
for the results. This kind of validity is called internal validity.
External validity concerns the extent to which the results
obtained in research study hold true outside that specific study.
 Generalization from a sample to the general population.
 Generalization from one research study to another.
 Generalization from a research study to a real-world
situation.

3. Any factor that allows for an
alternative explanation for the
results is a threat to internal
validity.

4. EXTRANEOUS VARIABLES, CONFOUNDING VARIABLES AND
INTERNAL VALIDITY
 If a study includes a
confounding variable, then
there is an alternative
explanation and the
internal validity is
threatened.
 Therefore, the key to
achieving internal validity
is to ensure that no
extraneous variable be
allowed to become a
confounding variable.

5. Environmental Variables
 Size of room, time of day, or gender of the
experimenter
 No systematic differences in the general
environment can avoid confounding
variables and ensure the internal validity of
a research study.
Individual differences
 For research studies that use a different
group of individuals for each of the
treatment conditions being compared, the
concern is that there may be individual
differences between groups.
 The problem is called assignment bias and
is a threat to internal validity because it
allows two alternative explanations for any
differences observed between the
treatments.

6. TIME-RELATED THREATS TO INTERNAL VALIDITY
Time-
related
Variables
History
Maturatio
n
Instrumentati
on
Testing effects
Statistical
Regression

7. BALANCING INTERNAL AND EXTERNAL VALIDITY