January 4, 2017

Measurement variability

The range is the interval between the lowest and highest values within a data group.

Variance of a sample (s^2)
The variance represents the deviation from the mean, expressed as the square of the units used.

Standard deviation of a sample (s, SD)
The standard deviation provides an estimate of the degree of scatter of individual sample data points about the sample mean.

Standard error of the mean (SEM)
The SEM is an abstract concept. It is simply a quantification of the variability of the sample means. The SEM is properly used to estimate the precision or reliability of a sample, as it relates to the population from which the sample was drawn. The SEM does not provide an estimate of the scatter of sample data about the sample mean and should not be used as such.

1. Gaddis G, Gaddis M. Introduction to biostatistics: part 2, descriptive statistics. Ann Emerg Med 1990;19:309-315.