EVALUATION OF THE PRECISION OF SYSTEMATIC-SAMPLING - NUGGET EFFECT AND COVARIOGRAM MODELING

Systematic sampling designs are widely used in stereology. When an est imator of the total amount, Q, of the sampled variable is evaluated by such a procedure, the coefficient of error can be predicted by applyi ng Matheron's theory of regionalized variables. To evaluate the accura cy of the estimate of Q, it is necessary to study the behaviour of the regionalized variable and to model its covariogram. Histological. dat a with a low short-range variability and agronomic data with a pronoun ced nugget effect provided the biological material for extreme case st udies. Results show that the short-range variability, if present, cann ot be detected when only small samples are available. An underestimati on of the coefficient of error is then to be expected. We propose seve ral models of the covariogram, which can be used to test for the prese nce of a nugget effect. If a nugget effect is present, these models wi ll provide better estimates of the coefficient of error. If there is n o nugget effect a simplified method can be used and will provide relia ble estimates of the coefficient of error.