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, such as light cyan or light magenta, but for our purposes that's an unnecessary complication.)

You have to run the results through basic statistical formulas to determine what's called the "90 percent lower confidence bound." That number gets reported as the yield. —next: Understanding the Yield Numbers >