The Nonequivalent Groups Design
The Basic Design
N
N

O
O
X
O
O
Key Feature: Nonequivalent
assignment
What Does Nonequivalent Mean?




Assignment is nonrandom.
Researcher didn’t control assignment.
Groups may be different.
Group differences may affect outcomes.
Internal Validity
History
Maturation
Testing
Instrumentation
Regression to the mean
Selection
Mortality
Diffusion or imitation
Compensatory equalization
Compensatory rivalry
Resentful demoralization
N O X O
N O
O











Internal Validity
Selection-history
Selection-maturation
Selection-testing
Selection-instrumentation
Selection-regression
Selection-mortality
N O X O
N O
O






The Bivariate Distribution
90
80
Posttest
70
60
50
40
30
30
40
50
Pretest
60
70
80
The Bivariate Distribution
90
80
Posttest
70
60
50
40
30
30
40
Program
60
Group
70
has
80 a
pretest5-point pretest
advantage.
50
The Bivariate Distribution
90
80
Posttest
70
Program
group
scores
15-points
higher
on
Posttest.
60
50
40
30
30
40
Program
group
has
60
70
80 a
pretest5-point pretest
advantage,
50
Graph of Means
80
75
70
65
60
55
50
45
40
35
30
Comparison
Program
Pretest
Comp
Prog
ALL
pretest
MEAN
49.991
54.513
52.252
Posttest
posttest
MEAN
50.008
64.121
57.064
pretest
STD DEV
6.985
7.037
7.360
posttest
STD DEV
7.549
7.381
10.272
Possible Outcome #1
70
65
60
Comparison
Program
55
50
45
40
Pretest
Selection-history
Selection-maturation
Selection-testing
Selection-instrumentation
Selection-regression
Selection-mortality
Posttest

 (CG not growing)


(PG moving away, CG level)
More low-score PG dropouts
Possible Outcome #2
70
65
60
Comparison
Program
55
50
45
40
Pretest
Selection-history
Selection-maturation
Selection-testing
Selection-instrumentation
Selection-regression
Selection-mortality
Posttest

 (Both growing)


(Wrong direction)
More low-score dropouts
Possible Outcome #3
70
65
60
Comparison
Program
55
50
45
40
Pretest
Selection-history
Selection-maturation
Selection-testing
Selection-instrumentation
Selection-regression
Selection-mortality
Posttest

 (In PG only)


(In PG)
More high-score PG dropouts
not as likely
Possible Outcome #4
70
65
60
Comparison
Program
55
50
45
40
Pretest
Selection-history
Selection-maturation
Selection-testing
Selection-instrumentation
Selection-regression
Selection-mortality
Posttest

 (In PG only)


(In PG)
 More low-score PG
dropouts
Possible Outcome #5
70
65
60
Comparison
Program
55
50
45
40
Pretest
Selection-history
Selection-maturation
Selection-testing
Selection-instrumentation
Selection-regression
Selection-mortality
Posttest





