Two audiences, one screen, and why it serves neither

Where the fourteen came from

Lesson 1 left a question hanging. Nobody at Halewatte sat down and decided that a supervisor needs fourteen numbers. So where did they come from?

They arrived one at a time, and every one of them was asked for by somebody who was right.

The supplier provided six as a standard layout. Merchandising asked for order completion and days to the ship date, because merchandising is asked those questions daily and the screen was already there. Quality asked for defects per hundred units and the rework queue. Chandana asked for the plant machine-utilisation average because he was pricing a second buttonhole machine. Iresha added work in progress at the slowest station.

Nothing was ever removed, because removing a number means telling a colleague their question does not matter. Fourteen numbers is what you get when two audiences are served from one surface and only one of them is in the room when it is designed.

The two questions

Read down the two columns. There is not a single row where the two agree. This is not a case of one being a summary of the other. They disagree on scope, on period, on the comparison, on how often it refreshes, and on whether the thing should ever interrupt anybody.

A surface that satisfies both ends up with all nine lines on it, so the supervisor has to hunt. It ends up with day-to-date running totals, so the manager has something covering more than an hour. It refreshes every thirty seconds, so the supervisor gets something live. And it has no history at all, because a wall screen has nowhere to put it. Both audiences get a compromise and neither gets an answer.

The calculation that settles it

The clearest evidence is what happens to a day-to-date figure, which is the compromise the two audiences produce between "this hour" and "the last thirteen weeks".

Line 6 runs at 87.5% when it is running. Suppose it stops for twelve minutes and nothing else goes wrong.

The same event moves the number by 8.1 points in the morning and 2.2 points in the afternoon — a factor of 3.7, and the factor is not a coincidence. A day-to-date figure divides a fixed loss by the time that has passed, so it gets less sensitive in exact step with the day going on. At 09:40 the time that has passed is 130 minutes. At 16:30 it is 480. And 480 divided by 130 is 3.7.

A threshold on that number means something different at every hour of the day. Set it to catch the morning stop and it screams every morning at anything at all. Set it to be quiet in the morning and it cannot see an afternoon stop of any size. There is no correct value, which is why lesson 1's sentence could not be finished for it. That was not an oversight in the specification. It is a property of the number.

What each surface should hold

The supervisor's board. One line, four numbers, refresh under a minute, no history, one alert channel. That is lesson 1's rebuilt board and it fits on a quarter of the screen already hanging there. The three-quarters that are freed hold the hour's plan drawn as a simple bar, and a list of the last three stops with their causes. Both of those are about what she does next, rather than how she has done.

The manager's sheet. Nine lines, five columns, thirteen weeks of history, printed on a Monday and read sitting down. Nothing on it refreshes, nothing on it alerts, and it costs Iresha forty minutes a week — 34.7 hours a year — which buys the only surface in the factory where one line can be compared with another.

One number that belongs on both, and why

The two surfaces are not sealed off from each other. There is a relationship that connects them, and knowing it stops a factory measuring three things where two would do.

Line 6 had 612 shirts sitting between stations when Iresha counted at three o'clock, and the line makes 768 shirts in a day. Divide, and a shirt entering the line takes 0.797 of a day to cross it — 6.38 hours. The work content of the shirt is 22.0 minutes. So a shirt spends 17.4 times as long on the line as anybody is working on it, and every minute of that difference is a shirt lying in a bin.

That relationship is fixed. For a line running steadily, work in progress divided by throughput is flow time. Work in progress is the pieces sitting between stations; throughput is the pieces the line finishes per day; flow time is how long one piece takes to get from the first station to the last. Display any two of the three and the third is already settled.

Which two you display is the audience decision. Kumudu gets work in progress, because a pile at station 24 is something she can move a helper to this afternoon. Chandana gets flow time, because a line that takes 6.38 hours to cross is a line whose response to a late change is measured in hours, and that is a planning fact he uses when he commits to a ship date. Throughput sits on both, which is why it is the only number in the factory that everybody is entitled to.

What to do on Monday

1. Write your two questions down, one for each audience, in the form of the table above. Fill in all seven rows for both.
2. Circle every row where they differ. If it is more than two, you need two surfaces.
3. Build the supervisor's board first and make it smaller than feels comfortable. Everything you remove has somewhere to go.
4. Give the manager's surface a fixed day and a fixed length. A weekly sheet that arrives on Monday gets read. One that is available whenever you like does not.