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Comparison of means calculator

Sample 1

Sample 2


This procedure calculates the difference between the observed means in two independent samples. A significance value (P-value) and 95% Confidence Interval (CI) of the difference is reported. The P-value is the probability of obtaining the observed difference between the samples if the null hypothesis were true. The null hypothesis is the hypothesis that the difference is 0.

Required input

For both samples, you enter:

  • Mean: the observed arithmetic mean.
  • Standard deviation: the observed standard deviation.
  • Sample size: the number of observations in the sample.

Computational notes

The program first calculates the pooled standard deviation s:

Comparison of means: pooled standard deviation$$ s = \sqrt { \frac { (n_1-1) s_1^2 + (n_2-1) s_2^2 } { n_1+n_2-2 } } $$

where s1 and s2 are the standard deviations of the two samples with sample sizes n1 and n2.

The standard error se of the difference between the two means is calculated as:

Comparison of means: standard error of the difference between two means$$ se(\bar{x_1} - \bar{x_2}) = s \times \sqrt{ \frac{1}{n_1} + \frac{1}{n_2} } $$

The significance level, or P-value, is calculated using the t-test, with the value t calculated as:

Comparison of means: t-value$$ t = \frac {\bar{x_1} - \bar{x_2}} { se(\bar{x_1} - \bar{x_2}) } $$

The P-value is the area of the t distribution with n1 + n2 − 2 degrees of freedom, that falls outside ± t (see Values of the t distribution table).

When the P-value is less than 0.05 (P<0.05), the conclusion is that the two means are significantly different. Note that in MedCalc P-values are always two-sided (or two-tailed).


  • Altman DG (1991) Practical statistics for medical research. London: Chapman and Hall. Buy from Amazon
  • Kirkwood BR, Sterne JAC (2003) Essential medical statistics, 2nd ed. Oxford: Blackwell Science. Buy from Amazon

How to cite this page

  • MedCalc Software Ltd. Comparison of means calculator. (Version 22.021; accessed February 27, 2024)

See also

Recommended book

Book cover

An Introduction to Medical Statistics
Martin Bland

Buy from Amazon

This textbook is intended for medical researchers and includes the design of clinical trials and epidemiological studies, data collection, summarizing and presenting data, probability, standard error, confidence intervals and significance tests, techniques of data analysis including multifactorial methods and the choice of statistical method, problems of medical measurement and diagnosis, vital statistics, and calculation of sample size.