1. A manufacturing apparatus for a method of optimizing a sample plan for measuring an overlay of a second-print pattern formed in a photoresist coated on a wafer at a subsequent layer relative to a first-print pattern formed on said wafer at an earlier layer comprising: determining a field-clustering scheme for said second-print pattern relative to said first-print pattern wherein said wafer is subdivided into fields; selecting an initial sample plan to measure overlay data at certain sampling sites on said wafer and in said fields; establishing an initial model for overlay, said overlay being algebraically decomposed into a set of systematic overlay components and residual overlay components; and establishing an efficient model for said overlay from said initial model for said overlay comprising: constructing matrices comprising linear regression estimators to fit said measured overlay data to said initial model for said overlay; eliminating components that are redundant; and eliminating components that are highly-correlated.