A small number of cells are used to inoculate parallel cultures in a non-selective medium. The cultures are grown to saturation to obtain equal cell densities. The cells are plated onto selective media to obtain the number of mutants ("r"). Dilutions are plated onto rich medium to calculate the total number of viable cells ( "N"t ). The number of mutants that appear in the saturated culture is a measure of both the mutation rate and when the mutants arise during the growth of the culture: mutants appearing early in the growth of the culture will propagate many more mutants than those that arise later during growth. These factors cause the frequency ( "r" / "N"t ) to vary greatly, even if the number of mutational events ( "m" ) is the same. Frequency is not a sufficiently accurate measure of mutation and the mutation rate ("m" / "N"t) should always be calculated.