A critique of the practice of plotting data obtained in vivo on an 'hours after treatment' format

The erroneous and misleading conclusions that result when data obtained in vivo are plotted on an 'hours after treatment' format when compared to a 'time of day' or 'time of circadian period' format are illustrated. The mitotic index of the corneal epithelium and the amount of DNA synthetic activity in the tip of the tongue are the examples used. Animals were kept on a light-dark cycle with light from 06.00 to 18.00 CST. Treatment with an intraperitoneal injection of 0.2 ml of saline at 05.00 compared to treatment with saline at 17.00 resulted in no effect on either variable when the data were plotted on a 'time of day' format, i.e., the data from the group which received saline at 05.00 and the group which received saline at 17.00 were very reproducible. However, when the same data were plotted on the 'hours after treatment' format, the data were 180 degrees out of phase with each other. This resulted in many statistically significant differences between the two groups. These differences are artifactual when compared to the 'no effect' or 'no perturbation' situation seen when the data are plotted on the 'time of day' format.