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A graphical aid for determining sample size when comparing two independent proportions.

P Feigl

    Biometrics
    |March 1, 1978
    PubMed
    Summary

    This study introduces graphical aids for sample size calculations when comparing two independent proportions. These charts help researchers determine the detectable difference based on a known feasible sample size.

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    Area of Science:

    • Biostatistics
    • Statistical Methods
    • Clinical Trial Design

    Background:

    • Sample size calculation is crucial for comparing two independent proportions (P1 and P2).
    • Standard calculations require specifying P1, the minimum detectable difference (delta), significance level (alpha), and Type II error rate (beta).

    Purpose of the Study:

    • To present a graphical aid for determining sample size (n) in two-proportion comparisons.
    • To provide a tool for estimating the detectable difference (delta) when sample size is constrained.

    Main Methods:

    • Utilized isographs (curves of equal sample size) based on the asymptotic arc sine approximation.
    • Developed charts plotting P1 against delta for sample sizes ranging from 35 to 500.
    • Created eight charts for various combinations of alpha and beta.

    Main Results:

    • The graphical aid allows direct reading of the detectable difference (delta) for a given feasible sample size (n).
    • Isographs are plotted on a grid with P1 on the abscissa and delta on the ordinate.
    • The charts facilitate sample size estimation in practical research scenarios.

    Conclusions:

    • Graphical aids offer a practical alternative for sample size determination in two-proportion studies.
    • These tools are particularly useful when the feasible sample size is known beforehand.
    • Researchers can efficiently identify the minimum difference detectable with a given sample size and error rates.

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