Though Fig. 167 appears to represent a soHd model, there was in reahty no sohd model made in order to obtain this illustration. By using isometric paper a chart like Fig. 167 can be made with comparatively little work. Isometric paper has lines ruled on the paper right and left at an angle of 30 degrees to the horizontal. By selecting a vertical scale to portray by separate columns the number of girls represented in any square of Fig. 166, it becomes a comparatively simple matter to draw the illustration. The various necessary lines can be drawn freehand in pencil on top of the isometric co-ordinate ruling, until the chart is completed; then the various lines can be inked in to get the final effect seen in Fig. 167. The total number of girls represented in any column is shown by the figures at the top of the column.

Daia of \V. GarneiL in ihc Journal of the Royal Statistical Society, 1910

Fig. 167. Examination Marks Obtained by 9,396 School Girls in England

This illustration was made from the same data as Fig. 166. Here the number of girls in any square is shown

by the height of the vertical column drawn to scale. The arrangement of scales here is different from that in Fig. 166 as will be noticed by observing the direction

of the arrows

CORRELATION 207

The reader should note carefully that the scale arrangement of Fig. 167 is entirely different from that used in Fig. 166. In Fig. 166, the two zeros fall together as they ordinarily should do in chart work of this sort. In Fig. 167, however, the two scale zeros are not together and the reader is accordingly prevented from interpreting Fig. 167 directly from the location of dots seen in Fig. 166.