All measuring instruments give wrong readings!

By MUNGAI KIHANYA

The Sunday Nation

Nairobi,

22 November 2020

In the recent assessment tests administered by the Kenya National Examinations Council, there was an unfair question in the standard 8 mathematics test. Students were asked to draw a geometrical diagram using a ruler, a protractor and a drawing compass. The dimensions and angles were given and the pupils were required to measure the distance between two points in the final figure.

Two of the multiple choices given were 4.9cm and 5.1cm. However, many of the children measured the distance as 5cm and, therefore did not know whether to select 4.9cm or 5.1cm!

Now, considering the limits of accuracy of the measuring instruments available to the pupils, this was an unfair question. The smallest subdivision on standard ruler is one millimetre, that is 0.1cm. On a student’s protractor, it is one degree.

In the hands of an experience person, say a teacher or an architect, the margins of error of these instruments would be plus-or-minus 0.5mm and 0.5 degrees, respectively. This is for each single line or angle drawn.

Thus, if they are used in drawing, say a triangle, each of the three lines and three angles will have the above stated inaccuracies. So, any extrapolation from such a diagram can have an error of up to plus-or-minus 1.5mm and 1.5 degrees.

Therefore, it was wrong to give two choice that are only 2mm apart: 4.9cm and 5.1cm. The options should have been at least 3mm apart. If they had given, say, 4.9cm and 5.2cm, a student who gets 5cm on the ruler could easily pick 4.9cm since this is the closest value.

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Due to the covid-19 pandemic, we are having our body temperatures measured at an unprecedented frequency. Mine is being checked at least four times each day as I drop and pick my children from two different schools.

Many people have noticed that the readings from different times are not the same. Does this mean that your temperature keeps fluctuating?

This question brings one the greatest false assumptions that many people make: that measuring devices are exact. On the contrary; all instruments give incorrect measurements! What matters is the level of accuracy.

The now ubiquitous thermal gun is inherently highly inaccurate. Most have error margins of about plus-or-minus1 to 2 degrees celcius. Thus, when it shows a reading of say, 36.5, the correct value could be anywhere between.34.5 and 37.5. For this reason, experienced medical professionals do not use the contactless thermometres.