A test for the hypothesis of zero second-order interaction is proposed. The test is obtained by making use of conditional reference sets which are derived in Zelen [lq. This permits the derivation of an exact conditional test of significance for testing the hypothesis of zero secondorder interaction. The techniques used, in approximating the large sample distribution of this test, are extensions of those due to Gart [5] and Zelen [la. The test is applied to a numerical example. and his co-workers. This method is equivalent to like& hood ratio test and, like the classical chi-square and likelihood ratio test, it is based on asymptotic distribution theory. The method is described in [lo].The study of small sample properties in situations where asymptotic theory is used for obtaining the distributions of test criterion is very valuable. This needed and useful work, in the present context, is done by Odoroff Bartlett [l] proposed a significance test for the [la] and Berkson [ 2 ] . Odoroff compares small sample hypothesis of no interaction in 2 x 2 x 2 tables. Roy properties of tests for interactions in 2 X 2 X 2 and and Kastenbaum [15] proposed an extension of Bartlett's 3 X 2 X 2 tables. He examined each of the three tests, definition for T X s x t tables. These authors used the minimum logit chi-square, Pearson's chi-square and Pearson's chi-square statistic and suggested obtaining the likelihood ratio test with four methods of estimathe required maximum likelihood estimates iteratively. tion, the iterative maximum likelihood estimation and This involves rather excessive computations. Plackett three variants of non-iterative minimum logit chi-square [13] developed a statistic, for the purpose, which did estimates. The exact levels of the minimum logit chinot need to obtain estimates of the parameters under the square tests and Pearson's chi-square test are better restrictions imposed by the hypothesis. The easily approximated by the tabular chi-square distribution than obtained unrestricted maximum likelihood estimates are are the levels of the likelihood ratio test. The exact levels required to compute the statistic. Goodman [6] sug-of tests based on minimum logit chi-square estimates are gested a variation of Plackett's statistic that requires better approximated than levels of tests based on less computations. maximum likelihood estimates. Interactions in contingency tables are compared with Berkson [a] considered the problem of "no interaction" interactions in the analysis of variance by Darroch [4],under two situations, one in which the prevalent model and this has led to a comparatively simple method of is of logit additivity and the other of linear additivity. constructing the likelihood ratio test statistic for the I n the logit model he investigated the effect of two hypothesis of zero second-order interaction in three-estimation procedures, unrestricted minimum discrimidimensional contingency tables. Goodman [7] has given nation information (UMDI) and maximum likelihood a different definition of intera...