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

RESPONSE VARIABLES

Know What You Want to Measure

If you do not collect the values of cofactors, you will be unable to account for them later.

As whiplash injuries are a common consequence of rear-end collisions, there is an extensive literature on the subject. Any physician will tell you that the extent and duration of such injuries will depend upon the sex, age, and physical condition of the injured individual as well as any prior injuries the individual may have suffered. Yet we found article after article that failed to account for these factors; for example, Krafft, Kullgren, Ydenius, and Tingvall [2002], Kumar, Ferrari, and Narayan [2005], and Tencer, Sohail, and Kevin [2001] did not report the sex, age, or prior injuries of their test subjects.

Will you measure an endpoint such as death or a surrogate such as the presence of HIV antibodies? A good response variable takes values over a sufficiently large range so that they discriminate well [Bishop and Talbot, 2001].

The regression slope describing the change in systolic blood pressure (in mm Hg) per 100 mg of calcium intake is strongly influenced by the approach used for assessing the amount of calcium consumed (Cappuccio et al., 1995). The association is small and only marginally significant with diet histories (slope −0.01 (−0.003 to −0.016)) but large and highly significant when food frequency questionnaires are used (slope −0.15 (−0.11 to −0.19). With studies using 24 hour recall, an intermediate result emerges (slope ...