Is This Reading Unusual Calculator

The best month in eight was 52. It sits 2.41 sigma from the average, and this process normally runs between 33.5 and 54.

So it is not a signal. It is the same process doing what it does.

That is the useful answer rather than the boring one. Changing a process because of a reading inside its own limits means acting on noise, and the usual result is a process that is worse and a confident story about why it had to be done. It would have to reach 54 before it meant anything at all.

The other half of the same discipline is that ignoring a reading which IS outside the limits costs just as much — and happens more often when the reading is good news. The page flags both directions.

Sigma comes from the average moving range rather than from the spread of the whole series, which is the standard method for a run of single readings, and the reason matters. The spread of the whole series treats a slow drift as if it were variation, so a drifting process gets limits wide enough to hide the drift. The moving range only ever compares neighbours, so a drift shows up as points leaving the limits — which is what you actually want.

The table puts every period against the same limits, which usually settles the argument on its own: most readings are normal, because that is what normal means.

Eight readings is not twenty. Limits built from eight points are indicative rather than firm and they move about as you add readings, so the page computes them from what you have and says exactly that instead of implying a precision it has not got.

It has to be the same thing measured the same way. A change in how something is counted looks precisely like a change in the thing itself, and no arithmetic on this page can tell those two apart.

Save as many versions as you like and put any two side by side, figure against figure, with the better one on each line marked and the difference in its own column. That screen is the one that decides things, and it is the part a free calculator cannot do at all. The same measure before and after a change is the comparison this was built for.

One HTML file. No account, no subscription, no internet needed. Opens on Windows, Mac and Android; the readme explains the extra step on iPhone and iPad, where Apple blocks JavaScript in downloaded files and every tool of this kind is affected.

Everything lives in your browser on your device. Nothing is uploaded, which is why it works offline and why nobody else can see your figures — and it also means clearing your browsing data deletes it. The tool does not pretend otherwise: the header shows how long since your last export at all times, turns amber after three days and red after seven, and export is one click that produces a plain file you keep.