Interpretation of EDA Plots:
Flat and Equi-Banded, Random, Bell-Shaped, 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 non-drifting.
- 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.
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 bell-shaped, and
- the normal probability plot will be linear.
Plots Utilized to Test the Assumptions
Conversely, the underlying assumptions
are tested using the EDA plots:
If all four of the assumptions hold, then the process
is said definitionally to be "in statistical control".
- Run Sequence Plot:
If the run sequence plot is flat and non-drifting,
the fixed-location assumption holds. If the run sequence
plot has a vertical spread that is about the same over the
entire plot, then the fixed-variation assumption holds.
- Lag Plot:
If the lag plot is structureless, then the randomness
If the histogram is bell-shaped, 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.