Figure 11a. Process of subtracting background from a sample. In this case, the true background value is 25 and the true sample activity is 75. Both the background measurement and the sample measurement represent Poisson processes, and each has an associated dispersion about the true value. (a) Graph shows three typical sample and background calculations. In these examples, the net counts vary from 42 to 97. (b) Graph shows the distribution of the net counts as a dotted line. This line represents the distribution for the true sample activity. The width of the net counts curve is greater than that of the sample or background curve because the variability of each curve must be taken into account during the subtraction process. As a result, the net counts probability density function is no longer Poisson because the standard deviation for this distribution is greater than the square root of the mean.