SOCIETY OF THE UNIVERSITY OF ABERDEEN. 



CORRELATION. 



95 



If we possess statistics of two dimensions of a considerable 

 number of individuals we can plot them out on a chart, using the 

 two dimensions of each person to determine his position on the 

 chart, just as the position of a point on a plane is determined when 

 we know ,r, y, and the axes of origin. 



Let us take as an example the 364 Aberdeenshire peasantry 

 mentioned above, and let us plot them out on a chart, using the 

 length and breadth of the head as co-ordinates. The axes of the 

 co-ordinates are the mean length LOL' and the mean breadth BOB' 



X 



Lengths. 



of the whole group, and or, y, for each individual are the deviations 

 from the mean length and mean breadth. If a perpendicular be 

 erected at each point on the chart, of a length proportional to the 

 average frequency about that point, the surface passing through the 

 tops of all these perpendiculars is called a " surface of frequency ". 

 Sections by perpendicular planes parallel to LOL' of the solid 

 bounded by the surface of frequency will be normal curves, but the 

 peaks of these normal curves will, as a rule, lie on a line C'OC. The 

 inclination of this line to B'OB is called by Galton the regression of 

 the lengths on the breadths. The regression of breadths on the 



