Fig. 2.

Examples of data artifacts. a Artifact one: The limit of precision of the electronic drug monitor. By expanding the dose time axis on this bi-modal plot, so that we are looking just at a single 15-min interval, it is clear that the electronic drug monitor concatenated dose events captured over time and either rounded up or rounded down events that occurred in between these intervals. The smallest time interval that could be measured was just under 0.025 h (or ~1.5 min). This makes clear that there is a limit to the granularity of this particular system. b Artifact two: Systematic drift in dose time. Examples of systematic drift in dose time, clearly observed in these two subjects whose dose time adherence was exceptionally regular. The bottom subject changed his/her dose time at the mid point of the study, and yet the slopes of the drift artifact remained exactly parallel in both periods. In all cases, on this scale, the slopes on the uni-modal plots measured 5° from the Y-axis. Over a 12-month period, the error totaled up to ~0.2 h (12 min, or 720 s), or a gain of ~2 s per subject per day. c Artifact three: Trending to new a baseline dose time, cause unknown. Bi-modal plot of a subject with a linear shift in dose timing over a several week span. The shift is presumed to be an artifact, though it is possible that the subject in question intentionally shifted their dose-timing forwards by several minutes each day. The ‘drift’ artifact is also quite easy to see in this subject, even on a bi-modal plot. d Artifact four: Linear artifact, presumably due to malfunctioning EDM device. Bi-modal plot of a series of linear artifacts, followed by a period when no data were recorded. It was not possible to re-contact the subject to determine whether a battery failure occurred, or whether the battery was changed or a new device assigned in January 2007. Note also that around the middle of January 2007 this subject abruptly moved his/her scheduled dose time about 30 min earlier