Industrial processes such as the production of pills are often
controlled by periodically sampling and analysing the product. For instance,
one can take every 100th tablet and analyse how much aspirin is actually
present. The aspirin content should not be too low, for then the pill
does not work, but not too high either because that is too expensive.
The results of these analyses are depicted in a so-called
This chart is set up by analysing data from a period
in which the production process was functioning OK. The variation in
aspirin content (a standard deviation) during this period is called sigma.
The solid line represents the target aspirin concentration and the
dashed lines are the warning limits, plotted at 2 sigma. The dotted
lines are called action limits and are plotted at 3 sigma.
What would you do if you were the plant manager and the latest
analysis showed up outside the action limits?