Interpretation of EDA Plots:
Flat and EquiBanded, Random, BellShaped, and Linear

The four EDA plots discussed on the
previous page are used to test the underlying assumptions:
 Fixed Location:
If the fixed location assumption holds,
then the run sequence plot will be flat and nondrifting.
 Fixed Variation:
If the fixed variation assumption holds, then the vertical
spread in the run sequence plot will be the approximately
the same over the entire horizontal axis.
 Randomness:
If the randomness assumption holds, then the lag plot will
be structureless and random.
 Fixed Distribution:
If the fixed distribution assumption holds,
in particular if the fixed normal distribution holds, then
 the histogram will be bellshaped, and
 the normal probability plot will be linear.

Plots Utilized to Test the Assumptions

Conversely, the underlying assumptions
are tested using the EDA plots:
 Run Sequence Plot:
If the run sequence plot is flat and nondrifting,
the fixedlocation assumption holds. If the run sequence
plot has a vertical spread that is about the same over the
entire plot, then the fixedvariation assumption holds.
 Lag Plot:
If the lag plot is structureless, then the randomness
assumption holds.
 Histogram:
If the histogram is bellshaped, the underlying distribution
is symmetric and perhaps approximately normal.
 Normal Probability Plot:
If the normal probability plot is linear, the underlying
distribution is approximately normal.
If all four of the assumptions hold, then the process
is said definitionally to be "in statistical control".
