A note on a "replication" of Rock's one-triallearning experiment

PETER M. WENDEROTH, University of Sydney, Sydney 2006, Australia

Breckenridge & Kooker (1969) reported a repUcation of Rock's (1957) experiment in which mean errors to criterion for a dropout condition (DOC) did not differ significantly from mean errors to criterion for a control group (Cl), who learned an unchanging PA list by repetition. Breckenridge and Kooker claimed that these results do not provide evidence for one-trial learning because they depend upon acceptance of the null hypothesis that mean errors are the same for DOC and Cl. In the second part of their study, Breckenridge and Kooker tested the recall of iterns previously learned. They found higher recall in Group Cl and this was taken to support an incremental theory.

Breckenridge and Kooker's apriori argument against acceptance of a null hypothesis is unwarranted. When such an argument is to be made, it is generally claimed that the null hypothesis has been accepted against an unreasonable alternative. In Rock's first study, the mean errors to criterion were 17.9 and 17.2 for Groups Cl and DOC, respectively. The reported standard deviations (9.9 and 8.6) suggest that a mean difference of about 10 errors would have been detected with Type I and Type 11 error rates of 0.05 (Guenther, 1965). Similarly, Rock's second experiment would have detected a difference of the order of 15 errors with

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the same error rates and, in that case, the group means were 26.7 and 29.2. Thus, it is likely that Rock accepted his null hypothesis against an unreasonably large alternative.

The study reported by Breckenridge and Kooker, however, is open to the same criticism. That study included a third group (C2) who leamed lists finally learned by Group DOC. The authors reported that mean errors to criterion were 26.41,36.00, and 41.94 for Groups C2, Cl, and DOC, respectively. Statistical tests indicated that (1) Cl = DOC, (2) Cl = C2, and (3) C2 * DOC. These tests are not orthogonal and contain an intern al contradiction: If (1) and (2) are true, then C2= DOC.

Contradictions may occur whenever more than a - 1 tests are performed on the means of a groups. Contradictions do not necessarily occur in such cases bu t this depends partlyon the power of the tests. With equal sampie sizes and fixed Type I and Type 11 error rates, the three null hypotheses tested by Breckenridge and Kooker would have been tested against a common alternative, Ö (Guenther, 1965). Since the means plot in a linear fashion and since only the difference between the extreme groups was rejected, this suggests that the power of the tests was not sufficient to detect the smaller differences.

There are at least two alternative approaches that could have been used in the first part of the experiment. If the

three treatments, Cl, C2, and OOC, had been assumed to be quantitatively equally spaced, tests of linear and quadratic trend could have been performed. In fact, Test 3 is the test oflinear trend and Tests 1 and 2 combined form a test of quadratic trend: C2 + DOC - 2(CI). The analysis by Breckenridge and Kooker indicates that significant linear trend would have been found with no residual quadratic trend. Hence, the conclusion would have been that the means were ordered in the form DOC > Cl> C2. However, the assumption of quantitatively equal spacing of the treatments is questionable.

A more acceptable and consistent approach would have been to specify two planned contrasts for both parts of the experiment: DOC - C2 and DOC + C2 - 2(CI), with fIXed Type I and Type 11 error rates and fIXed alternative Ö, with the latter specified in units of the population standard deviation.

In brief, it is probable that Breckenridge and Kooker's Group OOC performed at a lower level than did Group Cl, both in terms of errors to criterion and mean items recalled. Whether this is the case or not cannot be deterrnined from the report of the experiment. If it is the case, the results of the two parts of the experiment are consistent but the experiment does not replicate the findings generally attributed to Rock.

REFERENCES BRECKENRIDGE, R. L., & KOOKER, E. W. On

Rock's one-trial learning controversy. Psychonomic Science, 1969, 15,313-314.

GUENTHER, W. C. Concepts of statistical inference. New York: McGraw-Hill, 1965.

ROCK, I. The role of repetition in associative learning. American Journal of Psychology, 1957,75,186-193.

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