Abstract. We obtain an estimate on the average cardinality of the value set of any family of monic polynomials ofis the average second moment on any family of monic polynomials of F q [T ] of degree d with s consecutive coefficients fixed as above. Finally, we show that, where V 2 (d, 0) denotes the average second moment of all monic polynomials in F q [T ] of degree d with f (0) = 0. All our estimates hold for fields of characteristic p > 2 and provide explicit upper bounds for the constants underlying the O-notation in terms of d and s with "good" behavior. Our approach reduces the questions to estimate the number of F q -rational points with pairwise-distinct coordinates of a certain family of complete intersections defined over F q . A critical point for our results is an analysis of the singular locus of the varieties under consideration, which allows to obtain rather precise estimates on the corresponding number of F q -rational points.