Re: Accuracy vs Precision of #s

On Aug 11, 11:47 pm, "don" <d...@xxxxxxxxx> wrote:
"Gerry Myerson" <ge...@xxxxxxxxxxxxxxxxxxxxxxxxx> wrote in message

news:gerry-B14E7E.13300612082008@xxxxxxxxxxxxxxxxxxxxx

In article <g7qu0l$d2...@xxxxxxxxxxxxxxxxx>, "don" <d...@xxxxxxxxx>

If your scale says you weigh 153.49872 pounds, when in fact you weigh
about 200 pounds, your scale is very precise but not very accurate.

If your scale says you weigh 200 pounds, when in fact you weigh 198.73
pounds, your scale is fairly accurate, but not very precise.

Precision is about making distinctions;
accuracy is about getting it right.

Thanks - I like that analogy very much!

A real world example. At a former oil refinery, my company
measures the water & oil depth at various wells every quarter
(there is a lake of oil floating on top of the water table
underneath the former refinery).

Their tape measure is marked to a hundreth of a foot, but
the accuracy is often wrong by feet. This can be due to
many things, such as when the probe actually detects
oil/water/bottom, probe touching bottom and then allowed
to tilt, tape sticking to well pipe, etc.

Plotting all the readings of bottom depth usually produces
a bell curve. Good measurements produce narrow bells, bad
ones produce wide bells.

In this case, precision is generally a joke.
.

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