Effect of number of worms seeded on the Fitness Assay. A) Color of the FCs represent the number of worms. Colored dots represent the time-points. Grey area shows the time where the slopes of the curves are most distinctive for the number of worms. B) The slope of the linear relation between worm-number and relative OD is the steepest at 72 and 96 hours. Therefore the time frame between 24 and 96 hours is most distinctive for the number of worms when the slope of the curves is used. C) Number of worms plotted against slope of the relative OD between 24 and 96 hours. The variation in decrease of OD per hour is between approximately -0.025 and -0.0175 when 20 +/- 5 are taken. The maximum possible variation by different worm number is +/- 0.0025 OD per hour. From the linear relation of the slope and the number of worms one can calculate 'a comparable number of worms'. We explained the slope by the number of worms with a linear model. The standard deviation of the effect of the number of worms on the slope divided by the mean effect of the number of worms on the slope give a factor that can be used to determine a comparable number of worms. For the linear model this factor was 0.29. For example if 1sd deviation from the mean slope is accepted for a mean of 20 worms any number between 20 -/+ 5.8 can be used.