Different strata of data may be readily visible on a
run chart when examined with the perspective that
different processes may be overlaid on the chart. The
decision on how to disaggregate the data into homogenous
pieces is best made by someone with a profound
understanding of the processes and measures in question.
The question of data homogeneity is a common one
in statistics, and it should be thoughtfully considered.
The prescription for mixing and stratification is
both fundamental and challenging, since expertise in
measurement, statistical thinking, the processes, the
methodologies, and the application domain is needed
to understand what the data mean. Understanding the
data must precede sophisticated analyses.