Common Errors in Statistics (and How to Avoid Them), 4th Edition

INTERPRETING COMPUTER PRINTOUTS

Many of our reports come to us directly from computer printouts. Even when we are the ones who have collected the data, these reports are often a mystery. One such report, generated by SAS PROC TTEST is reproduced and annotated below. We hope our annotations will inspire you to do the same with the reports your software provides you. (Hint: Read the manual.)

First, a confession: We have lopped off many of the decimal places that were present in the original report. They were redundant as the original observations only had two decimal places. In fact, the fourth decimal place is still redundant.

Second, we turn to the foot of the report, where we learn that a highly significant difference was detected between the dispersions (variances) of the two treatment groups. We will need to conduct a further investigation to uncover why this is true.

Confining ourselves to the report in hand, unequal variances mean that we need to use the Satterthwaite’s degrees of freedom adjustment for the t-test for which Pr > |t| = 0.96, that is, the values of RIG for the New and Standard treatment groups are not significantly different from a statistical point of view.

Lastly, the seventh line of the report tells us that the different in the means of the two groups is somewhere in the interval (−0.05, +0.05). (The report does not specify what the confidence level of this confidence interval is and we need to refer to the SAS manual to determine that it is 95%.)

 The ...