When we desire to compare any two large statistical groups, we may compare median with median, quartiles with quartiles, and octiles with octiles; or we may proceed on the method to be described in the next paragraph but one.

The most convenient fractional parts are the middle (giving the median), the outside quarters (giving the upper and lower quartiles), and similarly the upper and lower octiles or deciles.

The Greeks prescribe the median or middle vein to be opened, and so much blood to be taken away as the patient may well spare, and the cut that is made must be wide enough.