Free statistical calculators
Comparison of means calculator
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.
The program first calculates the pooled standard deviation s:
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:
The significance level, or P-value, is calculated using the t-test, with the value t calculated as:
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).
- Altman DG (1991) Practical statistics for medical research. London: Chapman and Hall.