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Sampling: comparison of two proportions

Command:    

 Sampling
Next selectComparison of two proportions

Description

Calculates the required sample size for the comparison of two proportions. The sample size takes into account the required significance level and power of the test (see Sampling: Introduction).

Required input

  • Type I error - alpha: the probability of making a Type I error (Alpha-level, two-sided), i.e. the probability of rejecting the null hypothesis when in fact it is true.
  • Type II error - beta: the probability of making a Type II error (Beta-level), i.e. the probability of accepting the null hypothesis when in fact it is false.
  • First proportion (%): hypothesized proportion in the first sample.
  • Second proportion (%): hypothesized proportion in the second sample (the hypothesized difference with the first proportion is considered to be biologically significant).

Example

In the example you are interested in detecting a difference between two proportions of a least 15. You expect the two proportions to be equal to 75 and 60. For Alpha-level you select 0.05 and for Beta-level you select 0.10.

After you click the Calculate button, or press the Enter key, the program will display the required sample size, which is 200 in this example. This means that you will need 200 cases in each group, i.e. 400 cases in total.

A table shows the required sample size for different Type I and Type II Error levels.

Literature

  • Bland M (2000) An introduction to medical statistics, 3rd ed. Oxford: Oxford University Press.
  • Pocock SJ (1983) Clinical trials. A practical approach. Chichester: John Wiley & Sons.

See also