Single-Group Threats to Internal
Validity
The Single Group Case
Two designs:
The Single Group Case
Two designs:
Administer Measure
program outcomes
X O
The Single Group Case
Two designs:
Administer Measure
program outcomes
X O
Measure Administer Measure
baseline program outcomes
O X O
The Single Group Case
Alternative
Two designs: explanations
Administer Measure
program outcomes
X O Alternative
explanations
Measure Administer Measure
baseline program outcomes
O X O
Alternative
explanations
Example
Compensatory education in math for 1st
graders
Pre-post single group design
Measures (O) are standardized
achievement tests (at start of grade 1
and start of grade 2; Forms A & B)
History Threat
Pretest Program Posttest
O X O
Any other event that occurs between
pretest and posttest
For example, kids pick up math
concepts watching Sesame Street
Maturation Threat
Pretest Program Posttest
O X O
Normal growth between pretest and posttest.
They would have learned these concepts
anyway, even without program.
Testing Threat
Pretest Program Posttest
O X O
The effect on the posttest of taking the
pretest
May have “primed” the kids or they may have
learned from the test, not the program
Instrumentation Threat
Pretest Program Posttest
O X O
Any change in the test from pretest and
posttest
For example, change due to different
forms of test, not to program
Mortality Threat
Pretest Program Posttest
O X O
Nonrandom dropout between pretest
and posttest
For example, kids “challenged” out of
program by parents or teachers
Regression Threat
Pretest Program Posttest
O X O
Group is a nonrandom subgroup of
population.
For example, mostly low-math kids will
appear to improve because of regression to
the mean.
Regression Threat
Pretest Program Posttest
O X O