Creating meaningful charts and graphs for safety related issues can be hard, not to mention confusing. It doesn’t have to be. In this article I will share a technique that may help bring some clarity to the situation.
The Wrong Approach
First, let’s assume a company tracks the number of accidents per month. For the sake of an example let’s use this totally fabricated data set:
- Jan-07: 0
- Feb-07: 1
- Mar-07 : 0
- Apr-07: 0
- May-07: 0
- Jun-07: 1
- Jul-07: 1
- Aug-07: 0
- Sep-07: 0
- Oct-07: 0
- Nov-07: 0
- Dec-07: 1
- Jan-08: 0
- Feb-08: 0
- Mar-08 : 0
- May-08: 1
- Jun-08: 0
- Jul-08: 0
- Aug-08: 0
- Sep-08: 1
- Oct-08: 0
- Nov-08: 0
- Dec-08: 1
With this data, some may attempt to create a c chart. When they do they get something that looks sort of like this.
The upper control limit is 1.959 and the measure of central tendency is 0.304. Friends, this graph is pretty much useless. There is really no way of determining if things are getting better, worse, or staying the same.
A Better Approach
A better approach would be to somehow convert this data into a yearly rate. There are a number of formulas you can use to do this… but one I quite like is to simply count the days between accidents and multiply is by 365.
For example, the first accident (in our made up data set) happened on February 8, 2007 and the second accident occurred on June 16, 2007. This means there were 128 days between accidents.
So, our annual accident rate could be calculated as follows: (1 accident/128 days) * (365 days/year) = 2.85 accidents per year.
When we work the math for the rest of the data we see something like this.
- 8-Feb-07, first accident
- 16-Jun-07, 128 days between accidents, rate = 2.85
- 10-Jul-07, 24 days between accidents, rate = 15.21
- 17-Dec-07, 160 days between accidents, rate = 2.28
- 1-May-08, 136 days between accidents, rate = 2.68
- 6-Sep-08, 128 days between accidents, rate = 2.85
- 11-Dec-08, 96 days between accidents, rate = 3.80
Now, by using the rate data we can create an Individuals and Moving Range Chart (I-MR) that looks something like this.
This graph is far more powerful than the c chart. From this I-MR chart we can see that we had a blip early on but have since leveled off.
In other words things are pretty much staying the same. And since we’re talking about the safety of our workers the fact things are moving sideways – averaging around 5 accidents per year – is probably cause for strong action in most companies.
Not the Only Way
As I mentioned, this is not the only way to calculate an annual accident rate. With this said, I would be very interested to hear how your company goes about this.