Here x i 's are base 10 logarithms of concentrations of the compound where response measurements are recorded, and we assume that there are m unique x i values. At each x i , there are n i repeated measurements of response r i,j for j = 1, . . . , n i . Therefore, there are N c = m i=1 n i observations that are part of the concentration-response curve. The most common practice is to use the same number of replicates at each x i , such that n i = n for i = 1, . . . , m and N c = m • n.