A parametric parameter is selected, which has an upper specification limit and a lower specification limit. A storage percentile is determined. The storage percentile is equal to a product yield percentage if the number of the set of measurements greater than the upper specification limit exceeds the number of the set of measurements lower than the lower specification limit, and is equal to the product yield percentage subtracted from one hundred percent if the number of the set of measurements less than the lower specification limit exceeds the number of the set of measurements greater than the upper specification limit. A number of spatial regions on the wafer is designated. A first group of measurements from the set of measurements is obtained for a first spatial region of the spatial regions. A measurement closest to the storage percentile is stored.