6:1 preventive / corrective ratio

The 6-to-1 rule is used to check whether the frequency of a preventive task is well calibrated: on average, for every 6 times it is performed, a need for corrective work should appear. Enter the preventive tasks performed and the corrective tasks that came out of them in a period.

How it works

The rule comes from John Day Jr., engineering and maintenance manager at Alumax of South Carolina. According to the Life Cycle Engineering article, the ratio of preventive to corrective work orders should be about 6 to 1: preventive inspections should reveal some corrective work, on average, every 6 times they are performed.

The calculator computes ratio = preventive tasks performed ÷ derived corrective tasks, where derived corrective tasks are the corrective jobs that came out of those same preventive tasks (what was found and had to be repaired). And it reads as follows:

• Above 6:1: the preventive task is being done too often. Gridium gives an example: if after 6, 7 or 8 months no corrective work is needed, the monthly plan is probably too frequent.
• Below 6:1: the preventive task is not being done as often as needed.

The tolerance band is this site's own judgement. The sources set 6:1 as a reference, without bands; the calculator considers a ratio between 3:1 and 6:1 "calibrated", "short interval" above 6:1 and "long interval" below 3:1. With no corrective tasks, the ratio is undefined and is flagged as a short interval, but with little data it is best not to conclude: look at several periods.

An illustrative (made-up) example: a monthly task on a pump, 12 preventive tasks in the year with 2 derived corrective tasks, gives 6:1: calibrated. If the same 2 corrective tasks had come out of only 6 preventive tasks, the ratio would drop to 3:1 and it would be worth checking whether the interval is too long. It is a guide for each task or family of equipment: it does not replace technical judgement or the criticality of the equipment.

Sources