Page:Memory (1913).djvu/25

Rh more steep or more flat form depends on the nature of those observations. The more exact they are, the more will they pile up around the central value; and the more infrequent the large deviations, the steeper will the curve be and vice versa. For the rest the law of formation of the curve is always the same. Therefore, if a person, in the case of any specific combination of observations, obtains any measure of the compactness of distribution of the observations, he can survey the grouping of the whole mass. He could state, for instance, how often a deviation of a certain value occurs and how many deviations fall between certain limits. Or — as I shall show in what follows — he may state what amount of variation includes between itself and the central value a certain per cent of all the observed values. The lines +w and —w of our figure, for instance, cut out exactly the central half of the total space representing the observations. But in the case of the more exact observations of 1 b they are only one half as far from m n as in 1 a. So the statement of their relative distances gives also a measure of the accuracy of the observations.