A method of calibrating a model, the method comprising: Obtaining a reticle defining, at least in part, a test structure with a main feature and one or more sub-resolution features (34, 36) that are proximate to, and substantially parallel to, the main feature on the reticle; photolithographically patterning one or more substrates with the reticle to produce a plurality of patterned test structures; obtaining training data comprising: scattered radiation information from the plurality of patterned test structures, individual portions of the scattered radiation information being associated with respective process conditions being characteristics of a patterning process of the individual patterned test structures; and calibrating, using one or more processors, a model with the training data by determining a first ratio relating a change in one of the process characteristics to a corresponding change in scattered radiation information; wherein the one or more sub-resolution features include a first sub-resolution feature spaced away from the main feature on a first side of the main feature by a first distance sized to affect the corresponding patterned test structure, and wherein the one or more sub-resolution features include a second sub-resolution feature spaced away from the main feature on a second side of the main feature, different from the first side, by a second distance that is different from the first distance and is sized to affect the corresponding patterned test structure differently from the first sub-resolution feature; wherein the scattered radiation information comprises: a plurality of test conditions indicative of process characteristics of the photolithographic process used to produce the plurality of patterned test structures on the one or more substrates according to a focus-exposure matrix, with different patterned test structures being patterned under different test conditions, scatterometry pupil-intensity measurements of the plurality of patterned test structures, and data indicating which measurements correspond to which test conditions, and wherein the model infers a characteristic of a second photolithographic process from optical measurement of a second test structure patterned with the second photolithographic process. Obtaining a reticle defining, at least in part, a test structure with a main feature and one or more sub-resolution features (34, 36) that are proximate to, and substantially parallel to, the main feature on the reticle; photolithographically patterning one or more substrates with the reticle to produce a plurality of patterned test structures; obtaining training data comprising: scattered radiation information from the plurality of patterned test structures, individual portions of the scattered radiation information being associated with respective process conditions being characteristics of a patterning process of the individual patterned test structures; and calibrating, using one or more processors, a model with the training data by determining a first ratio relating a change in one of the process characteristics to a corresponding change in scattered radiation information; wherein the one or more sub-resolution features include a first sub-resolution feature spaced away from the main feature on a first side of the main feature by a first distance sized to affect the corresponding patterned test structure, and wherein the one or more sub-resolution features include a second sub-resolution feature spaced away from the main feature on a second side of the main feature, different from the first side, by a second distance that is different from the first distance and is sized to affect the corresponding patterned test structure differently from the first sub-resolution feature; wherein the scattered radiation information comprises: a plurality of test conditions indicative of process characteristics of the photolithographic process used to produce the plurality of patterned test structures on the one or more substrates according to a focus-exposure matrix, with different patterned test structures being patterned under different test conditions, scatterometry pupil-intensity measurements of the plurality of patterned test structures, and data indicating which measurements correspond to which test conditions, and wherein the model infers a characteristic of a second photolithographic process from optical measurement of a second test structure patterned with the second photolithographic process. a plurality of test conditions indicative of process characteristics of the photolithographic process used to produce the plurality of patterned test structures on the one or more substrates according to a focus-exposure matrix, with different patterned test structures being patterned under different test conditions, scatterometry pupil-intensity measurements of the plurality of patterned test structures, and data indicating which measurements correspond to which test conditions, and wherein the model infers a characteristic of a second photolithographic process from optical measurement of a second test structure patterned with the second photolithographic process.