Comparison of means (t-test)

Command:

Tests Comparison of means (t-test)

Description

Performs a t-test for the comparison of the means of two independent samples.

This test is not performed on data in the spreadsheet, but on statistics you enter in a dialog box.

Required input

• Sample 1, Sample 2: enter the oberserved mean, standard deviation and sample size for the 2 samples.
• Correction for unequal variances: select the t-test (assuming equal variances) or the t-test corrected for unequal variances (Welch test, Armitage et al., 2002). With the option "Automatic" the software will select the appropriate test based on the F-test (comparison of variances).

Results

The program displays the difference between the two means, and a 95% confidence interval for this difference. Next, the P value is displayed: when this P value is less than 0.05, the conclusion is that the two means are significantly different.

In the example, the mean 256 (standard deviation 22 and sample size 50) of the first sample, is compared with the mean 232 (standard deviation 32 and sample size 50) of the second sample. The difference between the two means is 24. The 95% confidence interval for this difference ranges from 13.1 to 34.9 (note that this interval excludes the value 0). The hypothesis test results in a P-value less than 0.0001, and the conclusion therefore is that, statistically, there is a significant difference between the two means.

In an optional Comment input field you can enter a comment or conclusion that will be included on the printed report.

Literature

• Armitage P, Berry G, Matthews JNS (2002) Statistical methods in medical research. 4^th ed. Blackwell Science.
• Bland M (2000) An introduction to medical statistics, 3^rd ed. Oxford: Oxford University Press.