Investigating Instrument Readings

An Instrumental Account of Instrumental Readings

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In many investigations, the contribution of instrumental readings plays a part. So just what do instruments tell us?  How accurate, reliable and precise are they? And what do these words mean anyway? 

We all use instruments every day, and think nothing of it. The most common perhaps being clocks, watches, thermometers and speedometers. But do we stop to think about the data they provide?

Take the simple car speedometer, on which we rely so much. Is it reliable and accurate? You might be surprised to learn it’s neither accurate nor reliable! Why is that?

Most car speedometers measure the speed of the car by recording the number of rotations of one of the wheels via the drive shaft. At its very simplest, this assumes that the diameter or circumference of the wheel is constant. It isn’t of course. The diameter of a wheel depends on the wear of the tyres. So, a brand new wheel will rotate more slowly than a well-worn one. Consequently, new tyres give slower speed-readings on the speedometer than well-worn ones.

And how do we know the speedometer isn’t accurate? Some claim it is because the police stop them for speeding when their speedometer reads otherwise. Sadly, this will only be true if the police speed camera is at fault, and that might be hard to prove! In truth, all speedometers are set to read higher than your true speed. This is for obvious reasons, to ensure no one is ever deceived into thinking they are keeping within the speed limit when they are not.  Do you want confirmation? Simply check your speed on a straight, level road with your sat. nav., if you are lucky enough to have one, and this will give you a truer indication of your actual speed.

How can you check the accuracy of an instrument? There is rarely an easy shortcut. You need to calibrate it in some way, by comparing it with a known standard. As an investigator, this is always a key question when considering instrumental readings: “When was the instrument last calibrated, and how has this been done?”

What do we mean by the reliability of an instrument? An instrument is reliable if it gives the same reading in the same circumstances. For example, if a reliable thermometer reads one degree above the true temperature, it will always do so at the same temperature. It is reliably wrong!

And what is meant by the precision of an instrument? We can get into very deep discussion on this matter, and there are a variety of different definitions, some of which relate closely to reliability. However, a very simple one is this: precision is an indication of the number of decimal places to which an instrument can be read. For example, a distance gauge that can read to a tenth of a millimetre is more precise than one that reads to only one millimetre.

So, if you are using instrumental readings to confirm a key aspect of an investigation, take care to consider the accuracy, reliability and precision of the instrument in question.