It appears that an aggregate calculated field or a level of detail (LOD) calculated field is required.
SUM([Incidence]) / SUM([Incidence]) / SUM([Incidence]) / SUM([Incidence]) / SUM([Incidence]) / SUM([Incidence] ([Population]) Instead of computing a ratio for each record and then adding the ratios, this algorithm computes the sums first and then divides them. When working with ratios, this is frequently required.
Furthermore, I assume your Population data don't change every day, therefore aggregating them with Sum() as seen above will inflate the denominator. In such instance, you'll need to use a LOD calculation to determine the population at a different level of detail. In that situation, a formula like this may be used.
MAX([Population]) / SUM([Incidence]) / FIXED [State], [County]: MAX([Incidence])