Here is the histogram for Process A.

We want to get all the results into the OK zone. One way to do this would be to move all the results to the left as shown below.

To get these results we would have to arrange that all the cars take a shorter time to travel the course. With this data the average is the tallest stack so these results have an improved average (because shorter time is better).

Because of the way the traffic lights work, some cars will always complete the course in a shorter time than average and some will complete the course in a longer time than average. This is shown on the diagram as the stacks of cars spread out on either side of the tallest stack. If, for some reason, we could not improve the average, another way we could get all the results into the OK zone would be to reduce the spread.

It is not immediately obvious how we would change the process to get the results we would like. It may involve removing some traffic lights or altering the red/green cycle times. The average might be improved by replacing all the cars with faster ones, but it is a short course so the performance of the cars might have very little effect on the outcome.

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