Nortel DMS-100 Series Application Manual page 392

9-4 How SPMS index values are calculated
The weighted average is a fraction. The top of the fraction is equal to the
sum of the basic index values multiplied by their weights. The bottom of the
fraction is simply the sum of the weights alone.
In calculating an aggregate index, any basic index is ignored if it displays an
NA instead of a numerical value. NA appears if the index is invalid in the
switch (example: line-related indices in a toll switch), or if the normalizing
factor is zero, indicating no activity on which to base an index for the given
time period. The composition of aggregate indices thus varies from one
switch to another, and to a much lesser extent, from one time period to
another.
Regardless of this variation, the aggregate index remains a valid summary of
the indices contributing to it. Operating companies using SPMS aggregate
indices in administrative plans may wish to compare switches only with
switches that show similar index composition (example: by grouping into
POTS local, Centrex, toll/tandems, etc.).
297-1001-330 Standard 11.04 June 1998