t-test for independent samples by groups

Select t-test, independent, by groups on the Basic Statistics and Tables - Quick tab to display the T-Test for Independent Samples by Groups dialog box, which contains three tabs: Quick, Advanced, and Options. Use the options on these tabs to compute t-tests for independent samples when the data are entered case by case, that is, to let each row in the data file represent one case (e.g., individual, respondent) and to let each column in the data file represent one variable or measurement (e.g., response to a questionnaire item). In this setup, a grouping variable (e.g., Gender) should be included to denote to which group each case belongs. Use these options if the data are arranged precisely in this manner.

If you have entered the data so that each variable represents the responses of one group, use the t-test, independent, by variables option on the Basic Statistics and Tables - Quick tab, instead. See the t-test for independent samples overview.

Variables. Click this button to display a standard variable selection dialog box. Select one grouping variable and a list of dependent variables for the comparison. Statistica computes the t-test for all variables in the dependent variable list, comparing the two groups that are identified by the two selected group codes (see below) in the grouping variable.

Code for Group 1; Code for Group 2. Specify the two codes that identify the two groups in the grouping variable that are to be compared in these fields. If you are not sure about the codes that were used in the grouping variables to identify the groups, double-click the edit field (or press the F2 key on your keyboard), and the Variable Code dialog box is displayed containing all integer codes and their alphanumeric equivalents found in the grouping variable in the current data file. You can select a grouping code in this dialog box (i.e., transfer it to the edit field) by double-clicking it.

Summary. Click this button to compute the t-tests for independent samples and display the results in a spreadsheet. The detail and formatting of the results depend on your selections on the Options tab.

Cancel. Click this button to close the dialog box without performing an analysis and return to the Basic Statistics and Tables dialog box.

Options. Click this button to display the Options menu.

By Group.  Click this button to display the By Group specification dialog box.

Select Cases. Click this button to display the Analysis/Graph Case Selection Conditions dialog box, which contains options to create conditions for which cases will be included (or excluded) in the current analysis. More information is available in the case selection conditions overview and syntax summary.

W. Click the W (weight) button to display the Analysis/Graph Case Weights dialog box, which contains options to adjust the contribution of individual cases to the outcome of the current analysis by "weighting" those cases in proportion to the values of a selected variable.

Weighted moments. Select this check box to specify that each observation contributes the weighting variable's value for that observation. The weight values need not be integers. This module can use fractional case weights in most computations. Some other modules use case weights as integer case multipliers or frequency values. This option is available only after you have defined a weight variable via the W option described above.

DF = W-1 / N-1. When the Weighted moments check box is selected, some statistics related to the moments (e.g., standard deviations and variances, skewness, kurtosis) can be based on the sum of the weight values for the weighting variable (W-1), or on the number of (unweighted) observations (N-1). The sums (and means), and sums of squares and cross products will always be based on the weighted values of the respective observations. However, in computations requiring the degrees of freedom (e.g., standard deviation, t-test, etc.), the value for the degrees of freedom can either be computed as the sum of the weight values minus one, or as the number of observations minus one. Moment statistics (except for the mean) are based on the sum of the weight values for the weighting variable when the W-1 option button is selected, and are based on the number of (unweighted) observations when the N-1 option button is selected. When the Weighted moments check box is selected, several graphics options will not be available. For more information on options for using integer case weights, see also Selecting a Weighting Variable.

MD deletion. When Casewise deletion of missing data is selected, Statistica ignores all cases that have missing data for any of the variables selected in the list. When Pairwise deletion of missing data is selected, all valid data points are included in the analyses for the respective variables (resulting possibly in unequal valid N per variable).