Observer-to-observer consistency in a time study

Why it matters

In a time study the observer is not only the person who reads the clock. The observer chooses the event at which the work starts and ends. If one observer takes the part touching the fixture as the start and another takes the hand leaving the part, the same footage can produce different times. Both records may be written correctly and still not represent the same work. An observer consistency review makes this distinction visible. The aim is not to rank people by speed or to pick the smallest time. It is to test whether a shared work definition can be applied the same way by different people. K01 and K02 here are not the workers doing the job; they are the codes of the observers marking the time boundaries.

To understand the source of a change in time, the same cycles must be compared. Work done at other times may really take different durations because of material position or short waits. Comparing the means of observers who measured different cycles mixes this real variation with the observer difference. In this lesson two observers each mark the same cycles on the same recording twice. In this way the cycle, observer and repeat information is kept in every row. Times are not sorted from smallest to largest; pairing is done on the cycle ID. Putting similar-looking times side by side is no substitute for comparing the same event.

Repeatability tests the same observer choosing the same event again; comparison between observers tests whether the definition carries from person to person. When the two repeats are very close, the observer means can still separate. In that case everyone may be applying their own interpretation consistently, and a shared interpretation has not yet been shown. Conversely, observer means can look equal while individual cycles show large differences in opposite directions. For this reason the overall mean alone is not reported. Alongside the signed difference, the absolute difference and the spread of the repeats are also examined. Deviations of opposite sign cancelling each other does not show that all boundaries agree.

This review is limited to the consistency of time marking. A measuring instrument's deviation from the true value, or splitting the total measurement variance into components, cannot be drawn from this lesson; those are covered in M24. Agreement between two observers is not, on its own, proof that both chose the right physical event. That the shared boundary closes the work scope correctly is verified separately. With Yamazo Studio, a time study can be done by marking work elements on video. Even when the tool is used, the definition and visibility of the boundaries and the responsibility for independent comparison stay with the measurement team. Besides numerical closeness, the extent to which the work is covered must also be protected.

Formula