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We propose a diagnostic to assess the influence of a unit ui in the sample s, selected from a finite population U={u1,....,uN} using a sampling design D, on θ^(s) as an estimator of a population parameter θ=θ(Y1,...,YN). We adjust the definition of conditional bias (Muñoz-Picardo et al., 1995), in order to obtain an influence diagnostic in survey sampling. Conditional bias as an influence measure of unit ui (ui∈s) on θ^(s), depends on Yi and on the sampling design, D. This is a distinctive feature of sample surveys. Only in particular cases is the proposed influence measure a case-deletion diagnostic. [ABSTRACT FROM PUBLISHER]