MedCalc

Sample size calculation: Single proportion

 Command: Sample sizeSingle proportion

Description

Calculates the required sample size for the comparison of a proportion with a given proportion. The sample size takes into account the required significance level and power of the test (see Sample size calculation: Introduction).

Required input

• Type I error - alpha: the probability of making a Type I error (α-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 (β-level), i.e. the probability of accepting the null hypothesis when in fact it is false.
• Proportion (%): the hypothesized proportion (considered to be biologically significantly different from the null hypothesis value), expressed as a percentage.
• Null hypothesis value (%): the null hypothesis value (a proportion, expressed as a percentage).

Example

In the example you consider a proportion of at least 70 to be significantly different from the null hypothesis value 50%.

For α-level you select 0.05 and for β-level you select 0.20 (power is 80%).

After you click Calculate the program displays the required sample size, which is 47.

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

Literature

• Machin D, Campbell MJ, Tan SB, Tan SH (2009) Sample size tables for clinical studies. 3rd ed. Chichester: Wiley-Blackwell.