Table 2.

Relative Hierarchy of Quasi-experimental Designs

Quasi-experimental Study Designs	Design Notation
A. Quasi-experimental designs without control groups
1. The one-group posttest-only design	X O1
2. The one-group pretest-posttest design	O1 X O2
3. The one-group pretest-posttest design using a double pretest	O1 O2 X O3
4. The one-group pretest-posttest design using a nonequivalent dependent variable	(O1a, O1b) X (O2a, O2b)
5. The removed-treatment design	O1 X O2 O3 removeX O4
6. The repeated-treatment design	O1 X O2 removeX O3 X O4
B. Quasi-experimental designs that use a control group but no pretest
1. Posttest-only design with nonequivalent groups	Intervention group: X O1
	Control group: O2
C. Quasi-experimental designs that use control groups and pretests
1. Untreated control group with dependent pretest and posttest samples	Intervention group: O1a X O2a
	Control group: O1b O2b
2. Untreated control group design with dependent pretest and posttest samples using a double pretest	Intervention group: O1a O2a X O3a
	Control group: O1b O2b O3b
3. Untreated control group design with dependent pretest and posttest samples using switching replications	Intervention group: O1a X O2a O3a
	Control group: O1b O2b X O3b
D. Interrupted time-series design*
1. Multiple pretest and posttest observations spaced at equal intervals of time	O1 O2 O3 O4 O5 X O6 O7 O8 O9 O10

O = Observational Measurement; X = Intervention Under Study. Time moves from left to right.

^*In general, studies in category D are of higher study design quality than studies in category C, which are higher than those in category B, which are higher than those in category A. Also, as one moves down within each category, the studies become of higher quality, e.g., study 5 in category A is of higher study design quality than study 4, etc.