tt_d.define_groups_button = get_widget_assert (xml, "define-groups-button");
tt_d.groups_entry = get_widget_assert (xml, "indep-samples-t-test-entry");
tt_d.define_groups_button = get_widget_assert (xml, "define-groups-button");
tt_d.groups_entry = get_widget_assert (xml, "indep-samples-t-test-entry");