Biological data, quantitative or qualitative, collected by measurement or counting are very variable. No two measurements in man are absolutely equal, not even the means or proportions of two series in health or in disease are equal although we compare like with the like. Variability is essentially a normal character. In other words, occurrence of variability is a biological phenomenon. Statistical variance gives a measure of how the data distributes itself about the mean or expected value. Unlike range that only looks at the extremes, the variance looks at all the data points and then determines their distribution.
This topic covers: Variance: Biological Variability, Real Variability, Experimental Variability, Observer error, Instrumental error, Sampling defects or errors of bias, Measures of variability, Measurement of the spread of data-range, Mean Deviation, Standard deviation, Coefficient of Variation, Standard Error of the Mean (SEM).