The first step in testing the difference between means is to compute a difference score for each subject. The difference score is simply the difference between the two conditions. The difference scores are shown below:

Notice that the difference between the visual mean (319.5) and the auditory mean (297.5) is the same as the mean of the difference scores (22). This will always be the case: The difference between the means will always equal the mean of the difference scores. The significance test of the difference between means consists of determining whether the mean of difference scores is significantly different from zero. If it is, then the difference between means is significant. The procedure for testing whether a mean is significantly different from zero is given in the section "Tests of µ, standard deviation estimated."Sub Visual Auditory Vis-Aud 1 420 380 40 2 235 230 5 3 280 300 -20 4 360 260 100 5 305 295 10 6 215 190 25 7 200 200 00 8 460 410 50 9 345 330 15 10 375 380 -5 Mean 319.5 297.5 22