The cephalic index is also greatly influenced by causes other than the length and breadth of head. A comparison of stature, height of face, and breadth of face with the cephalic index of the same series of Sioux Indians, is given in the following table:
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Although these values do not change quite regularly, they clearly show correlations between the three measurements which I selected and the cephalic index.
The index—like all other indices—is a complex value depending, as it does, on two measurements. In order to gain an insight into its significance, it will be best to investigate the correlations of its constituent elements. The correlation of length and breadth of head, determined from a series of 923 male adult Indians of the Sioux, Ojibwa, and Crow tribes, is as follows:
| Group. | I | II | III | IV | V | VI |
| Length of Head (mm.) | 180-184 | 185-189 | 190-194 | 195-199 | 200-204 | 205-209 |
| Average Breadth of Head. | 153.8 | 153.8 | 154.8 | 156.2 | 157.8 | 159.4 |
This table shows that the average breadth of head of individuals whose length of head is very great or very small, differs little from the average breadth.
When we investigate the correlation of length and breadth of head with stature, it is found that the length of head is more influenced by stature than the breadth of head. Correlation between breadth of face and horizontal diameters of the head shows the two transversal diameters to be very closely correlated, while the length of head is more closely correlated with height of face. The following table illustrates these observations:
