Question

Using Excel, calculate the median delay time per week and construct a run chart using the data below. Compare the delay times per week to the median. Display your chart. Question: Does this process show common or special cause variation? Is this something that the medical director should be concerned with? Would it be productive to investigate and eliminate the cause (whether it is common or special)? Why or why not?

Week	Average Delay Time (in days)
1	34
2	30
3	35
4	32
5	28
6	26
7	29
8	28
9	35
10	26
11	34
12	31
13	28
14	40
15	26
16	32
17	31
18	30
19	33
20	35

Answer 1

By using excel, median of the delay time is 31. (MEDIAN is the function used to calculate median)

Run chart -

Now, we have to count number of runs. run is a sequence of consecutive points which all on the same side of the median line. A run can be a single point if both the previous & subsequent points are on the opposite side of the median line. Ignore points that lie exactly on the line.

If the number of runs are more or fewer than expected indicates that there is special cause(assignable cause) in the process otherwise common cause (Non-assignable cause or random effects).

Table provided below for the expected number of runs -

We have total 20 data points, so the number of runs should between 6 & 15.

To find out runs, we will take 1 for score above median & 0 for score below median.

So, run becomes -

101100001010101011

So, there are 13 number of runs.

So, Runs is between 6 & 15, hence there is commom cause variation.

There are common cause variation in delay time which can be reduced by process improvement activities but not eliminated. So, medical director should be concerned with common causes such as poor design, measurement error, quality control error, poor working conditions e.g. lighting, noise, dirt, temperature.

Common causes are constantly active within the system & random error in system. It is not easy to identify cammon causes as they are natural & random. So, one may be able to identify the cause & can reduce it by process improvement activities. But, common causes can't be elimnated from the process. So, in case of common cause, it may or may not be productive to investigate & improve the process.