Individual Differences: Threats to Internal Validity for Studies
Comparing Different Groups
Personal characteristics that can differ from one individual to another are
known as individual differences. Examples include differences in height,
weight, gender, age, IQ, and personality. Because no two people (or animals)
are identical, individual differences are a part of every research study. 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. For example, if you select two individuals
and measure reaction time, one person will be faster and one will be
slower; if you measure age, one person will be older and one will be younger;
and so on. However, if individuals are assigned to treatment conditions so
that the faster people are consistently assigned to one treatment and the
slower people are consistently assigned to another, then you have a problem.
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. Specifi cally, it is possible that the scores in one
treatment are higher than the scores in another treatment because there are
real differences between the treatments, or it is possible that the differences
in scores are caused by the fact that the participants in one treatment are
faster (or smarter or older or more motivated) than the participants in the
other treatment.