My first professional job after graduating from college was a Junior Industrial Engineer tasked with time study duty and implementing methods improvements. Over the course of several years, I completed thousands of time studies with my trusted stopwatch. Since those days, I have learned several different approaches to setting the standard time like using the average time method, using the lowest repeatable time method and using the shortest observed time method. So which method should you use set a standard time?
My IE training in establishing a valid time standard was drilled into me to capture a repetitive method, rate the work pace, record the proper number of cycles, red circled non-standard times and take the average time as the standard. For example, if 10 cycles were recorded as follows in seconds (50, 53, 49, 47, 49 54, 46, 51, 65, 52) I would have red circled (not counted or excluded) the 65 second time and calculated the standard time for this task as 51 seconds (rounding up to the largest second). Also, I would have added a PF&D allowance (Personal Fatigue and Delay) boosting the time even higher but lets leave it out for this example.
When taught how to do a time observation to set the standard time in a kaizen event, the method was a little different. The observed work was set to a repetitive method and the lowest repeatable time was established as the standard time. Using the same times recorded in the above example, I would establish the standard time as 49 seconds.
Following the teachings of Taiichi Ohno, he believed that the best standard time is the shortest observed time. Again, using the same 10 time observation above, the standard time would be set at 46 seconds. Ohno stated that we should focused on the method used to complete the task in shortest time , trying to repeat it exactly and eliminating the causes in the other observations that prevented the operator from repeating the shortest time.
So which is the standard time in our example, 51 seconds, 49 seconds or 46 seconds?
Let’s look at taking the average observed time method resulting in the 51 second standard time. From the range of timed observations, we see that variation in the process exists. This variation could be from anything from dropping a screw, part mis-alignment, adjustments, location of parts, a miss feed, etc. Regardless of the cause, this variation is non-value added or a waste. So why would we include waste in our standard? What if the range was larger? Would we still accept this method? With our lean approach we should not accept this waste in our standard so it appears that using the average is not the best method.
As for the lowest repeatable time method, is this method of setting the standard time any better? In our example, it removes allowance for some of the waste however elements of waste would still be accepted. In our example, it is slightly better. But what if, based on our observations, the lowest repeatable time was above the average? In this case, it would include more waste in our standard. Again, this may not be the best method.
That leaves us with using the shortest observed time method as the best standard time. Some may argue with Taiichi Ohno saying that this one shortest recorded time is too strict, not repeatable, a fluke occurrence or even a small miracle. But instead of arguing against the merits of using the shortest time, we could put the same efforts into doing exactly what Taiichi Ohno tells us. Determine the reasons why the other times failed to reach the shortest time and eliminate them. Our efforts will be rewarded with the shortest time as the easiest time that can be repeated. Isn’t that core in our lean principles, seeing and eliminating waste?