Ttest

Format: Tt[est] [column]

Format: Tt[est] [p[aired] [column column]]

Student's t test is used to determine whether the values in one group are significantly higher than those in another group. If the option PAIRED (which may be abbreviated down to P) follows the command then a paired t test will be performed, otherwise an unpaired test. For the unpaired test the values all lie in one column, and two logical expressions must be entered to specify the conditions which define the two groups. For the paired test two columns are compared and the measures in each row are taken to be paired.

Example:

Enter command -  tt
Enter column to test:    GHQ
Enter condition for first group:  SEX=1
Enter condition for second group:  SEX=2

Output:

Studying C15 (GHQ)
Mean 18.84 if SEX=1
Mean 15.30 if SEX=2
Unpaired t test, 98 degrees of freedom: t = 0.927
Two-tailed p = 0.3560 (Assumes equal variances)
Comparison of means, standardised normal deviate: d = 0.977
Two-tailed p = 0.3283 (Does not assume equal variances)

Two tests are performed. One assumes that although the means may differ between the groups, the variances do not. This compares the means of the groups according to a t statistic and outputs a two-tailed probability value for the difference between the two means to be as large as it is by chance. The second test does not make the assumption of equal variances. It takes the difference between the means to approximate a normal distribution and quotes the two-tailed probability for the difference to be as large as it is.

If the PAIRED option is selected then the values in two columns are compared in a pairwise fashion, to see if the mean difference between pairs is significantly less than or greater than zero.

Example:

Enter command - TTEST P C16 C17

Output:

Comparing C16 (HDA) and C17 (HDD)
Paired t test, 99 degrees of freedom: t = 0.068
Two-tailed p = 0.9462

Each row contains a pair of values, one in each of the columns specified. The mean of the differences between these values is taken to be a t statistic and the probability value quoted is the two-tailed value for a difference as large as this to occur by chance.

If confidence limits have been requested (using the LIMITS command) then upper and lower confidence limits for the true mean difference between the groups or pairs will also be output.