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preface to fifteenth edition

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GENERAL INFORMATION, CONVERSION TABLES, AND MATHEMATICS 2.119<br />

The geometric mean of a set of N numbers is the Nth root of the product of the numbers:<br />

N<br />

pxxx 1 2 3...x N<br />

(2.7)<br />

The root mean square (RMS) or quadratic mean of a set of numbers is defined by:<br />

N<br />

2 2<br />

i<br />

q i1<br />

2.3.4 The Normal Distribution of Measurements<br />

RMS px x /N (2.8)<br />

The normal distribution of measurements (or the normal law of error) is the fundamental starting<br />

point for analysis of data. When a large number of measurements are made, the individual measurements<br />

are not all identical and equal <strong>to</strong> the accepted value , which is the mean of an infinite<br />

population or universe of data, but are scattered about , owing <strong>to</strong> random error. If the magnitude<br />

of any single measurement is the abscissa and the relative frequencies (i.e., the probability) of<br />

occurrence of different-sized measurements are the ordinate, the smooth curve drawn through the<br />

points (Fig. 2.10) is the normal or Gaussian distribution curve (also the error curve or probability<br />

curve). The term error curve arises when one considers the distribution of errors (x ) about the<br />

true value.<br />

FIGURE 2.10<br />

The Normal Distribution Curve.

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