Boxplot Construction
Compute
the following summaries from the batch:
-
Q1 Lower Quartile
-
MED Median
-
Q3 Upper Quartile
Compute
a STEP as 1.5*(Q3-Q1)
Draw
a scale appropriate for the data
Draw
a box of some width along the scale
whose bottom at Q1
whose top at Q3
Draw
a line at MED across the box
Set
-
Upper fence at Q3+STEP
-
Lower fence at Q1-STEP
Draw
-
a line from the top of the box to the
"closest data point to the upper fence which is still below it"
-
a line from the bottom of the box to the
"closest data point to the lower fence which is still above it"
Draw
individually each point outside the fences.
Set
- Upper Outer fence at Q3+2*STEP
-
Lower Outer fence at Q1-2*STEP
Draw
Emphasize each point outside the Outer fences.
Warning
- Do not draw: the fences, the outer fences, the numerical values of summaries.
- Do not over burden the scale.
- When choosing the scale, consider handling differently a single (a couple) extremely
outlying observation.