Learning rewrites the template in place, which means a rate that climbs over an
evening has three explanations and the numbers cannot tell them apart: the
template got better, the sensor got wiped clean, or the user learned where the
sensor likes to be pressed. Jorijn named the second and third while the numbers
were still going up. All three move the same way on the same time axis, and the
first trend run had no way to separate them because no earlier version of the
template survived.
A snapshot makes the paired test possible. Measure the learned template, restore
the older one, measure again within the same session -- the sensor is equally
clean and the user equally practised across both halves, so the only thing that
differs is the template. A rate that falls back on restore is learning. A rate
that stays up is not.
Restore stops the daemon before swapping the files and brings it back with
learning off, because the trustlet caches the template in memory once loaded and
because a restored template that immediately starts learning again is not a
control.
Jorijn van der Graaf
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