It is necessary to bear in mind:
- That the ability to find the anthropometric points implies a certain knowledge of anatomy; it is a practical research, to be made under the guidance of a teacher, while the actual finding of the points as well as the taking of the measurements, should be left to the learner.
- That the manner of applying the instruments is not without effect upon the resulting figure: for example, if the compass is held horizontally in measuring the frontal diameter, the result is different from what it would be if the instrument were held vertically. If the compass is held by the extremities of the branches, the diameter is slightly different from what it would be if the compass was held by the handle. Accordingly, it is necessary to describe minutely how we are accustomed to hold the instruments.
- That the resulting figure differs according to whether or not the screw has been turned, or whether it has been read in position, or by approaching the instrument to the eye.
- That when an instrument is old, it registers different results from those it gave when new; consequently, it is necessary to verify it, before proceeding to take a series of measurements. Hence it is proper to state not only precisely what instrument is used, but also that the precaution has been taken to verify it.
But what is still more important is to find out one's own personal data.
If the same measurement is taken twice under precisely similar conditions, the same figure is hardly ever obtained both times; everyone, even the most experienced, has his own personal error. By practice the amount of this error may be steadily lowered, but cannot be eliminated. Constant figures are an evidence of dishonesty, of mere copying; they are almost certainly not authentic.
It is important to know one's own average error.
It is calculated as follows:
Let us suppose that successive attempts have resulted in the following figures relative to the same measurement:
9, 10, 11, 12, 8
The mean average of these numbers is