Quasi-Experiment

Kinds of Quasi-Experimental Designs

Quasi-experimental designs include, but are not limited to, (a) nonequivalent control group designs, in which the outcomes of those exposed to two or more conditions are studied but the experimenter does not control assignment to conditions; (b) interrupted time-series designs, in which many consecutive observations over time (prototypically 100) are [Page 899]available on an outcome, and treatment is introduced in the midst of those observations to demonstrate its impact on the outcome through a discontinuity in the time series after treatment; (c) regression discontinuity designs, in which the experimenter uses a cutoff score on a measured variable to determine eligibility for treatment, and an effect is observed if the regression line of the assignment variable on outcome for the treatment group is discontinuous from that of the comparison group; and (d) single-case designs, in which one participantis observed repeatedly over time (usually on fewer occasions than in the time series) while the scheduling and dose of treatment are manipulated to demonstrate that treatment controls outcome.

In such designs, treatment is manipulated, and outcome is then observed. Two other classes of designs are sometimes included as quasi-experiments, even though the cause is not manipulated. In (e) case control designs, a group with an outcome of interest is compared to a group without that outcome to see if they differ retrospectively in exposure to possible causes; and in (f) correlational designs, observations on possible treatments and outcomes are observed simultaneously, often with a survey, to see if they are related. Because these designs do not ensure that cause precedes effect, as it must logically do, they usually yield more equivocal causal inferences.

Historical Development

Most experiments conducted prior to the 1920s were quasi-experiments. For example, Lind (1753) described a quasi-experimental comparison of six medical treatments for scurvy; around 1850, epidemiologists used case control methods to identify contaminated water supplies as the cause of cholera in London; and in 1898, Triplett used a nonequivalent control group design to show that the presence of an audience and competitors improved the performance of bicyclists.

In 1963, Campbell and Stanley coined the term quasi-experiment to describe this class of designs. Campbell and his colleagues (Cook & Campbell, 1979; Shadish, Cook, & Campbell, 2002) extended the theory and practice of these designs in three ways. First, they described a larger number of these designs. For example, some quasi-experimental designs are inherently longitudinal (e.g., time series, single-case designs), observing participants over time; but other designs can be made longitudinal by adding more observations before or after treatment. Similarly, more than one treatment or control group can be used, and the designs can be combined, as when adding a nonequivalent control group to a time series.

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