Introduction
Clinical trials are longitudinal. They accumulate data sequentially through time. All participants cannot be enrolled and randomized on the same day. Instead, they are enrolled as they enter the study. It may take several years to enroll enough patients to meet sample size requirements. Because clinical trials are long term studies, it is in the interest of both the participants and the researchers to monitor the accumulating information for early convincing evidence of either harm or benefit. This permits early termination of the trial.

Group sequential methods allow statistical tests to be performed on accumulating data while a phase III clinical trial is ongoing. Statistical theory and practical experience with these designs have shown that making four or five interim analyses is almost as effective in detecting large differences between treatment groups as performing a new analysis after each new data value. Besides saving time and resources, such a strategy can reduce the experimental subject’s exposure to an inferior treatment and make superior treatments available sooner.

When repeated significance testing occurs on the same data, adjustments have to be made to the hypothesis testing procedure to maintain overall significance and power levels. The landmark paper of Lan & DeMets provided the theory behind the alpha spending function approach to group sequential testing. This paper built upon the earlier work of Armitage, McPherson, & Rowe,  Pocock, and O’Brien & Fleming. PASS implements the methods given by Reboussin, DeMets, Kim, & Lan to calculate the power and sample sizes of various group sequential designs.

Group Sequential Tests of Means
This module calculates sample size and power for group sequential designs used to compare two treatment means. The program allows you to vary the number and times of interim tests, the type of alpha spending function, and the test boundaries. It also gives you complete flexibility in solving for power, significance level, sample size, or effect size. The results are displayed in both numeric reports and informative graphics.

Group Sequential Tests of Proportions
This module calculates sample size and power for group sequential designs used to compare two proportions. The program allows you to vary the number and times of interim tests, the type of alpha spending function, and the test boundaries. It also gives you complete flexibility in solving for power, significance level, sample size, or effect size. The results are displayed in both numeric reports and informative graphics.

Group Sequential Tests of Survival Curves
This module calculates sample size and power for group sequential designs used to compare two survival curves. The program allows you to vary the number and times of interim tests, the type of alpha spending function, and the test boundaries. It also gives you complete flexibility in solving for power, significance level, sample size, or effect size. The results are displayed in both numeric reports and informative graphics.

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