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92   PROBABILITY:

where variability is normal, the Means of B lie practically in a straight line on the face of the Table, and observation shows they do so in most other cases. It follows that the average deviation of a B value bears a constant ratio to the deviation of the corresponding A value. This ratio is called the' Index of Correlation,' and is expressed by a single figure. For example : if the thigh-bone of many persons deviate 'very much' from the usual length of the thigh-bones of their race, the average of the lengths of the corresponding arm-bones will differ ' much,' but not 'very much,' from the usual length of armbones, and the ratio between this ' very much' and 'much' is constant and in the same direction, whatever be the numerical value attached to the word 'very much.' Lastly, the trustworthiness of the Index of Correlation, when applied to individual cases, is readily calculable. When the closeness of correlation is absolute, it is expressed by the number i-o ; and by o-o, when the correlation is nil.

(New words and ideas-Correlation and Index of Correlation.)

This concludes what I have to say on these suggested Object lessons. It will have been tedious to follow in its necessarily much compressed form,-but will serve, I trust, to convey its main purpose of showing that a very brief course of lessons, copiously illustrated by diagrams and objects to handle,