A method of reducing distortion in a glass sheet is described and comprises the steps of: forming a glass ribbon in a glass manufacturing process; separating a glass sheet from the glass ribbon, the glass sheet having a substantially flat surface; measuring a retardation through the surface of the glass sheet; defining a retardation parameter indicative of the retardation of the glass sheet; cutting the glass sheet into a plurality of sub-sheets; measuring a distortion of the sub-sheets; defining a distortion parameter indicative of the distortion of the sub-sheets; and determining a correlation between the retardation parameter and the distortion parameter such that the distortion parameter of sub-sheets of a subsequent glass sheet can be predicted based on the correlation.