It’s generally agreed that there are seven basic tools needed to get started in SPC (Statistical Process Control). Some are so basic many people use them without realising they’re getting into statistics (and their link to what we think of as statistics often seems quite tenuous). Different authors give tools differing names (and describe them in varying ways); my way is:
- Check Sheets. This can take several forms from a general tally sheet (the simplest being the classic movie scene where the prisoner has marked the cell wall to keep track of his or her days in confinement) to a detailed list of requirements to be addressed for a task (sometimes, though not always, in sequence – an example being a pilot’s pre-flight checklist).
- Bar Charts. I’ve heard some people refer to it as a bar chart when the bars run horizontally and a histogram when they run vertically – but, to be more technically accurate, the former is for discrete data items/counts, the latter is for a presenting a continuous range of values.
- Pareto Charts. In its simplest form, a bar chart with progressively increasing values. It’s used to identify the more significant factors is a situation. Sometimes referred to as the 80:20 rule (80% of problems come from 20% of the causes).
- Fishbone Diagrams. This is the classic cause and effect diagram popularised by Professor Ishikawa. There are several rules of thumb regarding cause classifications: 6M, 4S and 4P are common – the important thing is to cover the options.
- Flow Charts. This is a diagram of the process flow; there are various formats that can be used and each can be quite simple or (often, over-) complicated.
- Scatter Diagrams. Sometimes called a run diagram or correlation chart, this is a graphical representation of value points on an x-y graph to try and identify trends or correlations.
- Control Charts. These chart process variation, usually over time. They can be quite simple (akin to a scatter chart where the measured parameter is the y coordinate and time (or sequence number) the x coordinate). However, it’s usual to set upper and lower control limits and there are various formulae to calculate these.
These seven aren’t all there are – the full list can appear endless – but understanding these will be a good foundation to anyone trying to master quality control. The links above will take you to brief introductions for each tool – but the emphasis is on “brief” as these pages can only give a flavour of them. However, this should give you enough to know what is worth exploring further when you need something. Most tools have whole book chapters (some whole books) devoted to them – and you could do a lot worse than getting hold of Juran’s Quality Control Handbook.
And this is only the basic set – there are many more…