Even the hour of the day has an influence upon the figures. It is known that none of us has the same ability to perform our various tasks at all the different hours of the day; for instance, it is not a matter of indifference whether we ask the pupils in a school to solve a problem at one hour of the day rather than at another. This is true of all occupations, and hence also of anthropometry; there are certain hours of the day at which fewer errors in measurement will be made, independently of the state of fatigue.
Consequently, it is well to know this individual datum, and to tell at what hour and in what environment the measures have been taken.
The figures are of more value if they have been compared with the results of other observers; it is necessary, after we have found our own average error, to select, for the purpose of verifying our results, some other observer, of similar experience to our own, and whose personal error is also known.
Here it is necessary to take into consideration still another factor—one's personal susceptibility to suggestion. If we have confidence in the person through whom we verify our figures, we are inclined to obtain figures equal to his own. We have only to compare our earlier figures with those since we began to use him as a test, in order to see whether, and to what extent we are influenced by suggestion. Hence, to obviate this danger it is necessary to obtain our respective figures without communicating them to each other.
It will also be necessary to take precautions not to be influenced by suggestion under any other circumstances. For instance, we are in hopes, while taking a series of measurements of school children, that we shall be able to prove that the heads of the more intelligent are larger than those of the less intelligent. In order that the figures shall be free from alterations due to suggestion, it is necessary that the measurer, while actually taking the measurements, shall be unaware which children are better and which are worse, from the intellectual point of view.
The personal error cannot be calculated in regard to a single measurement and then applied to all the others, but it must be worked out anew for every separate measurement; it oscillates variously, as a matter of fact, in relation to the longer and shorter diameters, the cranial measurements, and the measurements of the trunk and the limbs.
We are sufficiently skilled to take measurements when we have attained for measurements of cranial diameters a mean error of from 1 to 2 mm., for the vertical cranial diameter one of 4 mm., and for the stature, one of from 5 to 6 mm.
Finally, in anthropometry, theory is of no value without a long and intelligent practice, constituting an actual and personal education in anthropometric technique.
All anthropometric figures have a relative value dependent upon the extent of this education in the individual investigator.