A one-tailed test is appropriate when a directional difference is implied in the hypothesis being tested (i.e., "Group A will score higher than Group B"). A two-tailed test, on the other hand, tests the hypothesis that the two groups are "different", regardless of the direction of the difference (i.e., "Group A and Group B perform differently" on a certain measure).
While StarStat provides both 1- and 2-tailed probabilities, significance test results are displayed based on 2-tailed probabilities.
NOTICE Documentation and original code copyright 1995-2011 by DataStar, Inc. (Waltham, MA). Reproduction of material for non-commercial purposes is permitted, without charge, provided that
suitable reference is made to StarStat and DataStar, Inc. by including this notice intact.
Neither StarStat nor its documentation should be modified in any way without written permission from DataStar, Inc.
StarStat was previously known as Starware/Stat.
DISCLAIMER StarStat is provided "as is" without warranty of any kind. The entire risk as to the quality, performance, and fitness for intended purpose is with you. You assume responsibility for the selection of the test and for the use of results obtained.
More information on Z-Tests
More information on T-Tests
More information on Sample Precision
NOTICE
Documentation and original code copyright 1995-2011 by DataStar, Inc. (Waltham, MA). Reproduction of material for non-commercial purposes is permitted, without charge, provided that suitable reference is made to StarStat and DataStar, Inc. by including this notice intact.
Neither StarStat nor its documentation should be modified in any way without written permission from DataStar, Inc.
StarStat was previously known as Starware/Stat.
DISCLAIMER
StarStat is provided "as is" without warranty of any kind. The entire risk as to the quality, performance, and fitness for intended purpose is with you. You assume responsibility for the selection of the test and for the use of results obtained.
References