American National Standard Institute (ANSI) recommendations
are very similar:
"…preferred displacement is between 2.0 and 4.0 mm (0.08 and
0.16 inch)… the maximum force to depress the keys, expressed
in Newtons (N)...shall range between 0.25 N and 1.5 N. The
preferred key force is between 0.5 N and 0.6 N."
From the user point of view, the more proficient the user,
the lighter the touch can be. If too little force is required,
however, extra keys may be struck accidentally, and if too
much force or displacement is required, some keys may fail
to be actuated because the user did not press hard enough.
Also, excessive force induces finger fatigue. There is evidence
that optimum force/displacement characteristics of a key require
a steadily increasing force as the key is depressed until
contact is made. Immediately beyond that point, the force
is sharply changed so that users can easily "feel" when the
key has been pressed sufficiently.
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