Running the Numbers  It's easy to read more into the yield
numbers determined by the ISO/IEC standard than they actually
mean. The first thing you need to know is that they don't
represent a guaranteed number of pages you'll get from every
individual cartridge—not even if you're printing only the set of
pages defined by the standard. Basically, the stated yield is a
statistical prediction that if you pick a cartridge at random
and follow the procedures in the ISO/IEC standard, you'll get at
least that number of pages more than 90 percent of the time.  To
calculate the yield for what the ISO/IEC standard calls a
primary cartridge—black or tricolor, for example—you start with
the individual results from a minimum of nine cartridges. More
precisely, you need results from a minimum of three sets of
three cartridges tested on each of three printers, to take
variations among both individual printers and cartridges into
account. (The standard defines a different approach for what it
calls supplementary cartridges, s