Here, instead, we exploit the fact that the average plaque growth is approximately linear in time between 36 and 60 h. This allows us to extract a novel measure, the plaque velocity, which is the slope in the linear regression of the plaque radius to the 36, 48 and 60 h time points. The plaque velocity, unlike the plaque radius at a given time point, is a robust measure in that it takes into account plaque radius at several time points and is not affected by differences in the length of the delay period which precedes the period of linear growth. Using this method, the measured plaque growth was more rapid for the WT than the H275Y mutant, with a plaque velocity of 1:79+0:1d cell =h compared to 1:48+ 0:1d cell =h, where d cell is the diameter of one cell. Thus, using plaque velocity alone, it would appear that the WT strain has a replicative fitness advantage over the H275Y mutant.