SPCVisualsGuides
The questions people type into a search box at four in the afternoon with a number in front of them they do not trust. Each one answers the question first, says what the answer cannot tell you, and ends at a free calculator that runs it on your readings.
What is Cpk?
Cpk is the distance from your process mean to the nearer specification limit, measured in units of three sigma. Here is what it means, what a good value is, and the three ways it lies to you.
Read → CapabilityCpk vs Ppk — what the gap between them is telling you
Cpk uses within-subgroup sigma, Ppk uses the standard deviation of every reading. When they disagree, the disagreement is the finding — not a rounding artefact to be argued away.
Read → MeasurementMeasurement system analysis (MSA)
Before you trust a control chart or a capability number, you have to know how much of the variation you are looking at belongs to the gauge rather than the process. That is what MSA answers.
Read → CapabilityThe Cpk formula in Excel
The formula everyone puts in a spreadsheet returns Ppk, not Cpk. Here are both, done correctly, with the within-subgroup sigma Excel has no function for.
Read → VariationCommon cause and special cause variation
Every process varies. The question is whether the variation is the process being itself or something happening to it — and answering it wrong is how well-meant intervention makes a process worse.
Read → Control chartsWhat is an SPC chart?
An SPC chart is a time-ordered plot with limits calculated from the process itself — never from the specification. That one distinction is what makes it a control chart rather than a line graph with lines on it.
Read → Control chartsWhat is a Shewhart chart?
Shewhart chart is the proper name for what most people call a control chart. Here is where it came from, why the limits are at three sigma, and which of the family you need.
Read → Quality toolsHow to make a Pareto chart in Excel
Excel has had a built-in Pareto chart since 2016, and it is fine until you need it to behave. Here is the built-in route, the manual build, and the two mistakes that send an improvement team at the wrong defect.
Read → Comparing processesBox and whisker plot in Excel
Excel draws box plots natively, and the box it draws may not match the one your customer drew from the same numbers. The reason is the quartile definition, and there are three of them in common use.
Read → DistributionsNormal distribution curve in Excel
NORM.DIST draws the curve, and the scaling step is the one everybody misses. Then the harder question — a curve over a histogram is a picture, not a test of normality, and normality matters far more for Cpk than it does for a control chart.
Read → Quality toolsThe seven basic quality tools
Ishikawa argued that most quality problems can be solved with seven tools and no statistics beyond arithmetic. Here is what each one answers, when each is the wrong tool, and why the order you use them in matters more than the drawings do.
Read → Quality toolsThe fishbone (cause-and-effect) diagram
A fishbone diagram generates hypotheses. It does not test them, rank them or produce a scrap of evidence. Here are the categories, how to run one so it ends in a list of checks, and the two failures that make most of them worthless.
Read → LaboratoryWestgard rules explained
The six multirule QC rules, what each one detects, and why 1-2s is a warning rather than a rejection. Random error versus systematic error, and how to pick a rule set that does not cry wolf.
Read →How these are written
Each guide answers one question that somebody actually types into a search box, and answers it in the first paragraph rather than after nine hundred words of preamble about the history of quality management. Then it does the part most pages skip: it says what the answer cannot tell you. A Cpk of 1.42 is not a promise; a chart in control is not a chart making good parts; a fishbone diagram is a list of guesses until somebody checks one.
Where a calculation is involved, the formulas are given properly — including the Excel route, because most of this work is still done in a spreadsheet and a page that refuses to admit that is not useful to the person reading it. Each guide then ends at the free tool that runs the same calculation on your own readings, which is the only link on the page asking for anything.
Where to start
- New to control charts. What is an SPC chart, then common cause versus special cause — that pair covers most of what goes wrong in practice.
- Someone has asked you for a capability number. What is Cpk, then Cpk vs Ppk, where the gap between the two is the actual finding.
- You are doing it in a spreadsheet. The Excel guides give the formulas and the traps — the quartile definitions that make two correct box plots disagree, the Pareto that sends a team at the wrong defect, the scaling step in a normal curve everybody misses.
- You work in a lab. Westgard rules — what each multirule detects and why 1-2s is a warning rather than a rejection.
Why the answer usually arrives before the product does
Because most people reading these do not need software. They need the number explained once, correctly, so they can go back to the meeting. If that is you, the guides and the free tools are the whole offer and nothing further is expected. The product becomes relevant at exactly one point: when the calculation has to keep being right, on its own, for a process nobody is currently watching.
Prefer to skip the reading? The free tools compute all of it on your own data, with no signup and nothing stored.