All errors are calculated relative to the accurately measured result. I ran cpuchk.se using several sample intervals. This only collects long running processes, but keeps the load level from the cpuchk.se command itself to a minimum. Some sample output data is shown below. The first line shows the time of day, the number of processes, and the number of processes seen for the first time.
Subsequent lines show the error for each active process.
The last line shows how many processes were totaled. After looking at the data for individual processes for a while, I decided to concentrate on the summaries for each measurement interval. First I plotted both of them together in time sequence, then I plotted error as a function of CPU usage. The relationship is basically an inverse one, so I fitted and displayed an inverse relationship line. Not an ideal mix, but enough to investigate the effect of CPU speed and workload variations.