Figure 3.

Threshold of the ratio of costs of the first measurement over the subsequent ones (κ*) above which it is advisable to take as many repeated measurements as possible in the LDD case under the basic scenario. Otherwise, if the ratio of costs is less than κ*, the optimal is to take r_opt = 1. The basic scenario for the LDD response pattern assumes covariance structure CS(σ, ρ) for the response (θ = 0), no dropout (π_m = 0), constant exposure prevalence (p_ej = p_e, ∀j = 0, … , r) and CS(ρ_e) exposure correlation structure. The threshold takes the form ${\kappa }^{\star }=5+\frac{6(1-{\rho }_e)[2+(1-\rho \left){\rho }_e\right]}{(1+\rho ){\rho }_e}$. The column with constant value of κ* = 5 corresponds to a time-invariant exposure (ρ_e = 1).