The number of previous 15-minute intervals
for which data was collected.
An agent with PW capability must be capable of supporting at
least n intervals. The minimum value of n is 4, The default
of n is 32 and the maximum value of n is 96.
The value will be <n> unless the measurement was (re-)
started within the last (<n>*15) minutes, in which case the
value will be the number of complete 15 minute intervals for
which the agent has at least some data. In certain cases
(e.g., in the case where the agent is a proxy) it is
possible that some intervals are unavailable. In this case,
this interval is the maximum interval number for which data
is available.