Bagdonavicˇius and Nikulin (1997a) proposed to estimate the parameter γ by solving the estimating equations

U(γ, τ) = 0,

where the estimating function is given by the next formula

U(t, γ) = n∑ i=1

x(i)(u){dNi(u)− Yi(u)α(Hˆi(u, γ))dHˆi(u, γ)} =

{x(i)(u)− E˜(u, γ)} { dNi(u)− Yi(u)α(Hˆi(u, γ))γTx(i)(u)du

} ,

E˜(u, γ) = S˜(1)(u, γ) S˜(0)(u, γ)

, S˜(1)(γ, u) = n∑ i=1

x(i)(u)Yi(u)α(Hˆi(u, γ)).