Two-Sample Tests Results - Two-Sample Tests Tab

Comparing Samples

Select the Two-sample tests tab of the Two-Sample Tests Results dialog box to access options to display the results of the analysis.

Gehan's Wilcoxon test. Click the Gehan's Wilcoxon test button to display a spreadsheet with the results of Gehan's Wilcoxon test. Along with the test statistic, the spreadsheet will display WW, the sum of the scores for the first group where the score for each observation is the difference between R1 and R2; Sum, the sum of the squared scores across groups; and Var, the variance of WW. Note that STATISTICA applies a continuity correction to this test (see Lee, 1980). In addition, the spreadsheet will list all observations (survival times; note that censored observations are marked by a + sign), the group to which each belongs, the number of observations that are smaller than the respective observation (under the column header R1), and the number of observations that are larger than the respective observation (under the column header R2).

Cox's F-test. Click the Cox's F-test button to display a spreadsheet with the results of the Cox's F-test. The values T1 and T2 are provided in the output, which refer to the mean scores for the first and second groups, respectively, as defined by Cox (1964), as well as the test statistic which is the ratio of the two mean scores. In addition to the F-test, the spreadsheet will list all distinct failure times, the number of observations at risk at the respective failure time (under the column header R[I]), the multiplicity of each distinct failure time (under the header M[I]), the ratio of the two, and the Kaplan-Meier product-limit estimate of the survival function (in the right-most column of the spreadsheet).

Cox-Mantel test. Click the Cox-Mantel button to display a spreadsheet with the results of the Cox-Mantel test. The terms U and I that comprise the Cox-Mantel test statistic, which equals , are defined as follows:

where,

r2 is the number of failure times in group 2, m(i) is the multiplicity of ith failure time, r(i) is the risk set of ith failure time, A(i) is the proportion of the risk set that belongs to group 2 at the ith failure time.

In addition, the spreadsheet will list the risk set at each failure time (the number of observations whose failure time are at least this long or longer), and the proportion of observations in group 2 that belong to the respective risk set.

Peto & Peto's Wilcoxon test. Click the Peto & Peto's Wilcoxon test button to display a spreadsheet with the results of Peto & Peto's Wilcoxon test (Peto & Peto, 1972; see also Lee, 1980). Along with the test statistic, the spreadsheet will display WW, the sum of the scores for the first group; Sum, the sum of the squared scores across groups; and Var, the variance of WW. The score assigned to each observation is a function of the Kaplan-Meier product limit estimate of the survival function. In addition, the spreadsheet will list all observations (survival times; note that censored observations are marked by a + sign), the group to which each belongs, and the scores that are used in the computation of this test.

Log-rank test. Click the Log-rank test button to display a spreadsheet with the results of the log-rank test. Along with the test statistic, the spreadsheet will display WW, the sum of the scores for the first group; Sum, the sum of the squared scores across groups; and Var, the variance of WW. The assigned score for each observation is a function of the logarithm of the survival function (see Peto and Peto, 1972; see also Lee, 1980). In addition, the spreadsheet will list all observations (survival times; note that censored observations are marked by a + sign), the group to which each case belongs, and the scores that are used in the computation of this test.

Proportion surviving by group. Click the Proportion surviving by group button to display a spreadsheet with the comparative life tables for the two groups.