A single month's PPM is a snapshot. The useful information is in the trend, so plot customer PPM month by month on a simple line chart with the target drawn as a horizontal line. A run of months creeping upward tells you something is drifting even if each month is still under target. A single spike above target followed by a return to normal usually points to one bad lot rather than a process change.
Combining several months into a quarterly or annual figure has a trap. Do not average the monthly PPM values. Add up the defects across the months and divide by the total shipped across the same months, then multiply by one million. Months with bigger shipments carry more weight, which is correct, because more parts were exposed to risk.
Keep the chart on the wall where the production team sees it, with a short note on any month above target explaining what was found and what was done. The chart becomes the memory of the line.
Standard Procedure
- 1Record defects and quantity shipped for each month in a two-column table.
- 2Calculate and plot each month's PPM with the target as a reference line.
- 3For a quarter or year, sum the defect column and the shipped column before dividing.
- 4Annotate any month above target with the cause found and the action taken.
Lesson Summary
- ✓Trends matter more than single months; a steady climb is a warning even below target.
- ✓Combine periods with totals, not with an average of the monthly PPM figures.