Before control limits, capability indices or regression, there are three plain charts. Each answers one question.
| Chart | Question | Example |
|---|---|---|
| Run chart | Is it changing over time? | Days to close the books, month by month |
| Histogram | How spread out is it, and what shape? | Days late for every overdue invoice |
| Scatter plot | Does it move with something else? | Order changes vs. invoice errors, by customer |
Where it comes from
The histogram and scatter diagram are among the seven basic quality tools, and ASQ notes that some versions of the list include the run chart. The run chart is the simple version of the control chart: data in time order, with a center line, and no control limits. When you need limits, go to SPC.
How a bot helps
A bot can make all three from a spreadsheet in seconds. The useful part is what it writes underneath: the time order checked, the number of points, what changed in the period (a new system, a new hire, a price change), and what the chart can't say.
On a run chart, a few points above average mean nothing. A long run on one side of the center, or a steady trend, is a signal worth asking about. On a histogram, two humps usually mean two processes mixed together; stratify them. On a scatter plot, a pattern shows things move together. It doesn't show that one causes the other.
On the bots' own work
Run-chart a bot's weekly correction count before and after a skill change. If the drop is inside the normal week-to-week bounce, the change hasn't proved anything yet.
When to use it
After a check sheet has given you data, and before any statistical test. Light: an hour, mostly spent on what the chart can and can't say. If the question becomes "is this difference real?", that's the next step: experiments or a proper test.
Where bots differ
A bot will describe a pattern in any chart you give it, including random noise. Ask it to state the rule it used to call something a signal, and to say "no clear signal" when there isn't one.
Sources: ASQ: seven basic quality tools.
Back to Engineering tools for bots.
Short outline
Basic charts: short outline
What it's for
A run chart shows whether a measure is changing over time, a histogram shows its spread and shape, and a scatter plot shows whether it moves with another measure. Pick one by the question, and write down what it cannot show.
Columns
| Column | What goes in it |
|---|---|
| Question | Changing over time, spread and shape, or related to another measure |
| Chart | Run chart, histogram or scatter plot |
| Data | Source, period and number of points |
| Order checked | For a run chart: confirmed in time order |
| Events in the period | What changed and when: systems, people, prices, skills |
| Signal rule | The rule used to call something a signal, written before reading the chart |
| What it shows | The pattern, or "no clear signal" |
| What it can't show | Such as cause, or anything outside the period |
| Next step | Stratify, test, SPC, or stop |
Steps, each with its "done when"
- Pick the chart by the question. Done when the question is written down and matches the chart: time, spread or relationship.
- Check the data. Done when the source, period and count of points are stated, and a run chart's points are confirmed in time order.
- Write the signal rule first. Done when the rule for calling a signal is written before the chart is read.
- Make the chart and read it against the rule. Done when the reading says signal or no clear signal, and links any signal to events in the period.
- Say what it can't show and what's next. Done when the limits are written and the next step has an owner.
Common mistakes
- Reading a trend into a handful of points.
- Calling a scatter-plot pattern a cause.
- A histogram with two humps treated as one process.
- Changing the signal rule after seeing the chart.