Interpolation-based gradient-error bounds for use in derivative-free optimization of noisy functions

Abstract In this paper we analyse the accuracy of gradient estimates obtained by linear interpolation when the underlying function is subject to bounded measurement noise. The total gradient error is decomposed into a deterministic component—arising from the interpolation (finite-difference) approximation—and a stochastic component due to noise. Various upper bounds for both error components are derived and compared through several illustrative examples. Our comparative study reveals that strict deterministic bounds, including those commonly used in derivative-free optimization (DFO), tend to be overly conservative. To address this we propose approximate gradient error bounds that aim to upper bound the gradient error norm more realistically, without the excessive conservatism of classical bounds. Finally, drawing inspiration from dual real-time optimization strategies we present a DFO scheme based on sequential programming, where the approximate gradient error bounds are enforced as constraints within the optimization problem.