perfect measurement system no errors

Specifications are difficult. Both myself (Mike) and Polly Taylor lecture regularly on methods for nociceptive threshold testing and we often find that people are unsure about the terms in the datasheets for pieces of equipment that they might buy. Elsewhere on this website you will find our specs for MouseMet and so here is an explanation of the terms used, and some suggestions about what to look out for when choosing.

A Perfect Calibration: On the right is a calibration graph from a “perfect” piece of equipment, with the true value of whatever is being measured on the X axis and the indicated value on the Y. The quantity measured is arbitary (hence the axis title); it could be force, it could be temperature, it could be humidity. The point is that there is no difference between the true reading and the indicated reading at any point between 1 and 5 arbitrons.

The manufacturer could therefore reasonably claim that his device is calibrated and shows no errors between 1 and 5 arbitrons. Note firstly that we don’t know what happens below 1 arbitron and I deliberately didn’t put a blue dot at zero-zero to make this point; the device might have an offset or non-linear characteristic (more later) when it measures near zero and that the graph shown, if extrapolated, does not go through the zero-zero point. This has implications if you only intending to use the device near the bottom of its range; the quoted “accuracy” may not apply if you are below the bottom calibration point. It is not uncommon to quote an accuracy figure and to then note, in the small print, that the figure applies “down to 10% (or 5%) of the measuring range”. 10% of the measuring range in the case above is 0.5 arbitrons, or half way between the last calibration point and zero and as we will see, there are many effects which can degrade the first 10% of the range.

Real Force Numbers: An electronic von Frey system is a force transducer. If, for instance, the calibrated force range of the device is up to 100 grams force (often abbreviated to gf and also, incorrectly, to just g), then we should certainly check where the calibration points are before using it to measure forces lower than 10gf or 5gf. If the range of the device is greater (500gf or even 1000gf), we should be even more careful. We should also not be unduly placated by other quoted parameters such as repeatability and particularly, resolution. These numbers are relevant, but only part of the story.

Repeatability: To me, the most important number is one that is not generally quoted; it is the repeatability under real measurement conditions (it’s sometimes wrapped up with reproducibility, and there’s more about this further down). So what is repeatability?

Repeatability is the ability of a measurement system to indicate the same measured value each time it is given the same quantity to measure.
(my own definition)  

The point is that many measurement systems have very good repeatability (or, to put it the other way round, very low scatter or spread) when they are calibrated under favourable conditions. Staying with my example of the force transducer (because that’s what an electronic Von Frey system is), these are most easily calibrated against another, higher grade, force transducer or, more simply, using a set of known masses which you apply, one at a time, and record the indicated force. Easily demonstrated, with your kitchen scales and a bag of sugar; put the mass on a number of times and look at the variation in the number of grams indicated. Your scales will probably read exactly the same value each time, and so you would be entitled to quote a repeatability (at this force level) equal to the smallest change that your scales can indicate (this is the “Resolution” of the system).

And that’s fine for kitchen scales; they’re intended to be used on a bench. But now try holding the scales in your hand and putting the bag of sugar on…You’ll probably get a different answer each time now, and you’ll also be able to make the reading change by moving the scales up and down. The repeatability has degraded.

This, while an unfair test for the kitchen scales, is completely relevant to a force transducer that is intended to be handheld and used to apply a force to a rodent’s foot. The force transducer may show a very high repeatability while mounted on the bench, but that’s not how it will be used.

So how do we specify  MouseMet? We do it two ways. First of all we calibrate with masses, on the bench, (because if you can’t get that right then there’s no point in going near an animal) but we then also derive a repeatability figure (as a standard deviation from the mean) for baseline testing. This is the spread or scatter that the system should give, in the lab, with mice (or rats). This value has been derived from experimental data from several MouseMet systems in the field in combination with some mathematical error analysis.

—More to come, if you’ve read this far and need a quick answer to a measurement question, then please send me an email.