Each scanned vital gets a minimum level change and a spread floor at
about its clinical and device resolution, so a VO2 max estimate that
steps by 0.3 or a clean 2 bpm change no longer reads as a shift. A
series a straight line explains at least as well as a step is a drift,
not a dated shift. The magnitude is capped at 10 so the flattest series
cannot win the ranking.