BWOM: Accuracy or Precision?

This month’s Big Word of the Month post involves two words that are not very big but have very specific meanings in science.

Take a look at our collections of hygrometers/thermometers from the reptile room.

The electronic and analog meters are in very close agreement on temperature, all around 79°F. There is a little more disagreement in the measurements of relative humidity. The electronic ones are in the 20’s while the analog one is reading around 50%.

So which one is correct? We need to buy several more of these and we need to know which one is the best.

The best instrument for any measurement is one that is both accurate and precise. Accuracy is how close your measured value is to the actual value. Precision refers to how close together repeated measurements of the same value are.

Consider the following scenario:

We weigh the farmyard pig with one scale and get the following values: 146, 151, 138
Using another scale we take weights at the same time and get: 135, 136, 134

If the pig’s real weight is 145 pounds, which scale gave you better results? The seemingly obvious answer is the first scale because it was almost right once and off by 6 or 7 pounds the other times. However, the second scale would be much more useful if you are trying to track small weight changes in the pig. If a Veterinarian told us that the pig’s weight was fine but we need to note any changes of more than 5 pounds, the first scale would not be very useful. In this example the first scale is more accurate but less precise than the second one.

So where does that leave us with the reptile thermometers/hygrometers. Luckily, I know someone at Duke that has a very expensive and carefully calibrated growth chamber for plants. It measures temperature and humidity very accurately and precisely. I’m going to take all our hygrometer/thermometers to the chamber and see which one is best.

We’ll probably end up using one of the small electronic ones. It turns out that almost all cheap hygrometer/thermometers use the same electronics to measure humidity and the accuracy of that circuit is about plus or minus 4°F. Don’t discount the analog meter completely, it has two advantages. It does not require batteries and it has a calibration screw on the back so you can adjust the reading.

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