CHAPTER V.
NORMAL VARIABILITY.
Schemes of Deviations.-Normal Curve of Distribution.-Comparison of the observed with the Normal Curve.-The value of a single Deviation at a known Grade determines a Normal Scheme of Deviations.
Two Measures at two known Grades determine a Normal Scheme of Measures.-The Charms of Statistics.-Mechanical illustration of the Cause of the Curve of Frequency.-Order in apparent Chaos.
Problems in the Law of Error.
Schemes of Deviations.-We have now seen how easy it is to represent the distributionn of any quality among a multitude of men, either by a simple diagram or by a line containing a few figures. In this chapter it will be shown that a considerably briefer description is approximately sufficient.
Every measure in a Scheme is equal to its Middlemost, or Median value, or M, plus or minus a certain Deviation from M. The Deviation, or "Error" as it is technically called, is plus for all grades above 50°, zero for 50°, and minus for all grades below 50°. Thus if (±D) be the deviation from M in any particular case, every measure in a Scheme may be expressed in the
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