‘‘Duration’’ is defined as the time spent on a sub-function, i.e., the
difference between the requested time of two adjacent entries in
a session. We conjecture that, as the time spent on a sub-function
increases, the likelihood of the elder becoming interested in the
sub-function increases. Typically, an elder quickly jumps to another
sub-function if a sub-function is not useful. However, a quick jump
might also result from the short operating length of a sub-function.
Hence, normalizing ‘‘Duration’’ by the operating length of the subfunction,
that is, by the basic operating time of the sub-function,
is more appropriate. Eq. (2) is used to measure the ‘‘Duration’’ of
a sub-function,