The Cusum graph

Last time I discussed the possible flight from recording and suggested that not showing most, if not all, records in graphical form could be a major reason for it, writes John Gadd. One graph in particular, the Cusum ( cumulative sum) is very useful in this respect.

There are about 15 or so important measurements needed to run a tight pig business and Table 1 below suggests some of them. Individual producers and their advisers will have others, but no matter, they can all be designed by a programmer into the computers that we all have these days. It takes a computer no longer to print out a picture than it does to provide those eternally boring columns of figures which are such hard work.

The Cusum graph records periodic progress from a wide variety of critical events against the target fixed by the producer. I described this in my Targets and Targeting blog on May 13. In the cusum‘s case, the target is always the 45-degree bisector. The weekly, monthly and so on progress is plotted on the horizontal axis and the performance achieved for that period on the vertical axis The result appears either above or below this rising target line.

My example at Figure 1 shows performance against the successful service rate, but there are many other measures that can be recorded in the same way.

Recording graphically in this way has two main advantages:

While underachieving target is what concerns most people, the cusum can also show if overacheiving may become a future problem. For example, insufficient accommodation leading to the world-wide error of overstocking is one, as my diagram shows. The cusum will also forewarn of a developing problem. Sure, columns of figures will do the same, but it’s so much easier and quicker to notice it from a picture especially this sort of picture. In future computer programmers will, I hope, build in a statistical overlay with a green/amber/red urgency that can indicate what needs attention, and also go on to provide a drop-down menu of suggested corrective action if the traffic-light shows “red”.
Those using the cusum system and pinning up the more important of the weekly progressions on the staff restroom wall report how interested the staff become in the weekly progress as it reveals not only the results of their work, but how other departments’ results may be affecting them as well. This encourages weekly discussion with management and nips problems in the bud.

Table 1: Some essential records; many of them can be kept on a cusum graphs that forewarns of developing trends, while those marked * require progression graphs that will reveal a sudden decline