Pedagogical Anthropology, Maria Montessori [online e book reader TXT] 📗

Fig. 150.

Limbs.—In the case of the limbs also it is by no means easy or practicable to take many measurements. Consequently it should be sufficient to indicate that there are a great number of different measurements for every different segment of the limbs.

There are two principal instruments needed for this: a large compass with adjustable branches, for the long segments, and a small compass for the short segments. With the large compass we measure the length of the upper arm and forearm, the length of the thigh and shin, the length of the foot. With the small compass we measure the total length of the hand, its width, the length of the fingers and of the digital segments, etc.

The circumference of the limbs is taken with the ordinary metallic tape.

In order to fulfil the present-day scope of pedagogic anthropology, it is sufficient to take only a few measurements (the form and the head), but it is necessary to take them with great accuracy, and above all, to verify one's personal ability as a measurer, so that everyone who wishes to try the experiment may have a reliable method of testing himself. To this end it is necessary to know how to calculate one's own special personal error.

The Personal Error

In anthropometry, a knowledge of the anthropometric points, the instruments to apply to them, their use and their interpretation, is not sufficient. There is need of prolonged experience in accordance with the accepted method and under a practical guide.

As a matter of fact, the degree of accuracy with which a measurement is taken is always relative, no matter who takes it, but in the case of a person who has had no practice this relativity may present so wide a margin as to be practically useless.

To obtain an approximate figure of a measurement means nothing, unless the figure is supplemented not only by a statement as to which of the accepted methods was used in taking it, but also by a minute description of the manner in which this method was carried out.

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

(9+10+11+12+8)/5 = 10

Let us see how the values obtained differ in respect to 10:

9 10 11 12 8 10

-1, 0, +1, +2, -2 = differences from the mean average figure. We now take the average of these differences, disregarding the plus and minus signs:

(1+0+1+2+2)/5 = 6/5 = 1.2 = mean average error

The personal mean error is a datum that it is necessary to know in order to give value to any measurements that we may wish to give forth.

In taking the various test measurements for the purpose of calculating one's personal error, it is well to use the precaution of not taking them twice at the same sitting, but after an interval of time, not only so that all marks will have disappeared that may have been left upon the skin by the instrument in the act of measuring, but also that the preceding figure will have faded from our memory. Accordingly, the measurements should be repeated on successive days and if possible under the same conditions of time and place.

It is well to make a careful choice of the time and place, because these also have their effect upon the figures.

It will be observed that if the measurements are made in a well-appointed place, with a steady light, without noises, in short, without disturbing causes, the personal error is much more easily decreased, i.e., the measurements are more exact, because the measurer can better concentrate his attention.

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.

This is a case in which it may be said that the figures are worthless without the signature.

CHAPTER VIII
STATISTICAL METHODOLOGY

Having taken measurements with the rigorous technical precision that is to-day demanded by anthropometry, we should know how to extract from these figures certain laws, or at least certain statistical conclusions.

There are two principal methods of regrouping the figures:—mean averages and seriations.

Mean Averages.—Averages are obtained, as is a matter of common knowledge and practice, by taking the sum of all the figures and dividing the result