Disclosed method of measuring a parameter relating to a structure formed using a lithographic process, and more specifically focus or line edge roughness. The method includes measuring a structure having a dimension, e.g., a critical dimension, which is sufficiently large to enable radiation diffracted by at least one edge of said structure to be (e.g., individually) optically resolved. The method comprises obtaining an intensity metric from an image of the at least one edge and determining a value for said parameter based on the intensity metric.