A Meta-analysis of Pretest Sensitization Effects in Experimental Design

A meta-analysis of outcomes from 32 studies investigating pretest effects was conducted. All outcomes were computed as standardized differences between pretested and nonpretested groups. Eleven other variables were coded for each outcome. Initial descriptive statistics were indicative of differences between randomized and nonrandomized studies, so all further analyses were based on randomized group outcomes (n = 134). For all outcomes the average effect size was +.22, indicating the general elevating effect of pretest on posttest. Cognitive outcomes were raised .43, attitude outcomes .29, personality .48, and others about .00 standard deviations. Sixty-four percent of all effects were positive, and 81 percent of the cognitive effects were positive. Duration of time between pre- and posttesting was related to effect size, with effect size generally being small for durations less than a day or over 1 month. Year of publication, sample size, presence of experimental treatment, and sameness or difference of pretest and posttest were not significantly related to effect size. Researchers must continue to include pretest as a design variable when it is present, and to estimate its effect.