Question

Five data entry operators work at the data processing department of the Georgia Bank. Each day for 10 days, the number of defective records in a sample of 200 records typed by these operators has been noted, as follows:

Sample	1	2	3	4	5	6	7	8	9	10
No. of Defectives	19	22	28	12	14	8	25	10	16	6

1. What SPC chart is needed here?

2. Using z=3, establish upper and lower control limits for the control chart and judge if the process is in control.

Answer 1

Sample	No. of defects	p
1	19	0.095
2	22	0.11
3	28	0.14
4	12	0.06
5	14	0.07
6	8	0.04
7	25	0.125
8	10	0.05
9	16	0.08
10	6	0.03

pbar = sum of all p values / number of samples

=0.08

Here, p chart would be the most appropriate.

UCL = pbar + 3 [ pbar ( 1-pbar)/n]^1/2 = 0.08 + 3 x [ 0.08x0.92/200]^1/2 = 0.1375

LCL = pbar - 3 [ pbar ( 1-pbar)/n]^1/2 = 0.08 - 3 x [ 0.08x0.92/200]^1/2 = 0.0224

The process is out of control as one value is out of control limits.