PROC SHEWHART graphing bug for R-chart with subgroup size=1

[Quentin]

Hi All,

I'm getting an invalid control chart where the color band for an R-chart sometimes extends outside of the limits.  It looks like it's a graphing bug.  Seems to happen when there are  varying subgroup sizes, and one of the groups has a size of one.  I think it's trying to put a break in the band (because the control limits are undefined for an R chart with subgroup size=1), and something is going wrong in the graphing code.

When it happens, I get a chart where the band blot has crazy shapes, like:

I replicated on 9.4M4 windows and 9.4M6 linux, with below code taken from Shewhart docs (I replaced some of the values with missings).

data Disks;
  input Lot @;
  do i=1 to 6;
    input Time @;
    output;
  end;
  drop i;
  datalines;
1 8.05 7.90 .    .    .    .
2 8.03 .    .    .    .    .
3 8.00 7.94 7.97 7.95 8.00 8.01
4 8.00 8.06 8.06 7.99 7.97 7.96
5 7.93 8.01 8.00 8.09 8.06 8.02
6 7.98 7.99 8.01 8.09 8.00 7.97
7 8.00 7.94 7.93 8.03 7.93 8.08
8 8.01 7.98 7.98 8.07 8.05 8.09
9 7.97 7.96 8.01 8.11 8.06 8.07
10 7.93 8.03 8.03 8.00 7.93 8.03
11 8.00 .    .    .    .    .
12 7.98 7.93 8.01 7.97 8.02 8.00
13 8.06 7.93 7.98 7.98 8.02 7.96
14 8.05 7.98 8.05 7.99 7.95 7.99
15 7.94 8.01 7.97 8.04 7.91 8.03
16 8.03 8.03 8.02 8.06 8.00 7.97
17 8.03 7.94 8.05 8.05 8.04 7.94
18 7.99 7.99 7.86 7.99 8.06 8.03
19 7.95 7.96 7.99 7.96 7.94 8.12
20 8.03 8.07 7.98 7.97 8.00 8.04
21 8.04 7.90 8.03 8.02 7.98 7.97
22 7.95 8.05 7.98 8.01 7.97 8.15
23 8.06 8.00 8.03 8.02 7.99 7.95
24 7.97 8.02 8.00 7.96 7.96 8.00
25 8.12 7.97 7.99 8.09 8.05 8.00
;

ods graphics on ;
proc shewhart data=Disks;
rchart Time*Lot/markers;
run;

Wondering if anyone has seen this before, could replicate running above code, and agree it's a bug.

Thanks for thoughts,

--Q.

[ChrisHemedinger]

Quentin, you should definitely report this to SAS Technical Support. I cannot weigh in as to whether it's a bug. Others here might be able to, but regardless: reporting toward a possible fix or workaround is best tracked with Tech Support.

[Quentin]

If I specify cinfill=none to suppress the coloring of the area within the control limits, the plot looks fine.

[Quentin · Accepted solution]

Hi,

Tech support confirmed this is a bug [track SAS 7613523932], and said they would refer it to the developer. They also pointed out that the graph issue is specific to the graph being produced with ODS graphics.  If you turn off ODS graphics, SHEWHART will use traditional SAS graphics, and does not have the problem:

ods graphics off;
symbol v=dot;
proc shewhart data=Disks;
  rchart Time*Lot/markers;
run;
ods graphics on;

--Q.