Experimental DesignsExperimental Designs

Campbell & Stanley’s (1963) Campbell & Stanley’s (1963) three general types of three general types of experimental designsexperimental designs

Pre-experimentsPre-experiments Quasi-experimentsQuasi-experiments Full or true experimentsFull or true experiments

Key conceptsKey concepts Random assignment:Random assignment: assigning participants to the various assigning participants to the various

conditions in an experiment to achieve equivalent groups for conditions in an experiment to achieve equivalent groups for comparison purposes.comparison purposes.

Treatment condition:Treatment condition: administering the experimental administering the experimental manipulationmanipulation

Control condition:Control condition: no experimental manipulation or an no experimental manipulation or an alternative, benign manipulationalternative, benign manipulation

Pretest:Pretest: measurement of the dependent variable(s) prior to measurement of the dependent variable(s) prior to exposure to a treatment or control conditionexposure to a treatment or control condition

Posttest:Posttest: measurement of the dependent variable(s) following measurement of the dependent variable(s) following the treatment or control conditions.the treatment or control conditions.

notation for experimental designsnotation for experimental designsR = random assignment R = random assignment

N = no random assignmentN = no random assignment

0011 = pre-test (measurement) = pre-test (measurement)

0022 = post-test (measurement) = post-test (measurement)

XX11 = treatment, stimulus condition, or = treatment, stimulus condition, or

manipulationmanipulation

XX0 0 = control group (no treatment)= control group (no treatment)

one-shot case study: A group (usually intact) is administered a treatment and then measured or observed. No attempt is made to randomly assign subjects to condition, nor does the design provide for any additional groups as comparisons.

N X1 O1

one group, pre-test, post test design

N O1 X O2

static group comparison: only one of two intact groups is given the experimental treatment. At the end of the treatment, both groups are observed or measured to see if there is a difference between them as a result of the treatment.

X1 01

X001

N

N

intact groups, no pretest

Pre-experimental designsPre-experimental designs

Quasi-experimental designsQuasi-experimental designs

Pre-test, post-test, quasi-equivalent groups designPre-test, post-test, quasi-equivalent groups design

N0N011 X X O O22

N0N011 O O22

Times-Series designTimes-Series design

N0N011 0 022 0 033 04 04

Full Experimental DesignsFull Experimental Designs

pre-test/post-test equivalent groups designpre-test/post-test equivalent groups design

RR 0011 XX11 OO22

0011 OO22

post-test only equivalent groups designpost-test only equivalent groups design

RR XX11 OO11

OO11

Solomon four-group designSolomon four-group design

RR 0011 XX11 OO22

0022 OO22

XX11 OO22

OO22

a.k.a. two-group posttest only design

true/full experimental designstrue/full experimental designs

Solomon Four Group design:Solomon Four Group design: attempts to control for the attempts to control for the possible "sensitizing" effects of the pre-test or possible "sensitizing" effects of the pre-test or measurement by adding two groups who have not been a measurement by adding two groups who have not been a part of the pre-test or pre-measurement process.part of the pre-test or pre-measurement process.

RR 0011 XX11 OO22

0022 OO22

XX11 OO22

OO22

Factorial designs (may be full or quasi-Factorial designs (may be full or quasi-experimental)experimental)

Advantage: Allows the researcher to uncover Advantage: Allows the researcher to uncover interaction effectsinteraction effects 2 X 2 design2 X 2 design

– Two independent variablesTwo independent variables

– Two levels/values per variableTwo levels/values per variable

– Four conditions being comparedFour conditions being compared 3 X 2 X 2 design3 X 2 X 2 design

– Three independent variablesThree independent variables

– Three levels/values for one independent variable, two Three levels/values for one independent variable, two levels/values for the other two independent variableslevels/values for the other two independent variables

– Twelve conditions being comparedTwelve conditions being compared