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Sampling: Comparison of two means

Command:    

 Sampling
Next selectComparison of two means

Description

Calculates the required sample size for the comparison of two independent means. 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.
  • Difference: the hypothesized difference (considered to be biologically significant).
  • Standard deviation 1 : hypothesized standard deviation in the first sample.
  • Standard deviation 2 : hypothesized standard deviation in the second sample.

Example

In the example you are interested in detecting a difference between the sample means of a least 10. You expect the standard deviations in the two studies to be equal to 16. 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 54 in this example. This means that you will need 54 cases in each group, or a total of 108 cases.

Comparison of two paired samples

To calculate the sample size required for the comparison of two paired samples, see Sampling for single mean.

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.

See also