1. A method of correcting a mask pattern design, comprising: preparing a desired pattern and a mask pattern to form the desired pattern on a substrate; defining at least one evaluation point on an edge of the desired pattern; defining at least one process parameter to compute a transferred/formed pattern; defining a reference value and a variable range for each of the process parameters; computing, by the computer, a positional displacement for each of first points on the transferred/formed pattern corresponding to the evaluation point, the first points computed using the mask pattern and a plurality of parameter values obtained by varying the process parameters within the variable range or within the respective variable ranges, the positional displacement being displacement between the first point and the evaluation point; computing, by the computer, statistics of the positional displacements for each of the evaluation points; and outputting, by the computer, information to modify the mask pattern according to the statistics, wherein the computation of the positional displacement includes computing a light intensity at the evaluation point, wherein the computing the light intensity includes: preparing, by the computer, a mask pattern data defining a complex amplitude transmission distribution for the mask pattern; performing, by the computer, a Fourier transformation of the complex transmission distribution of the mask pattern; computing, by the computer, a mutual transmission coefficient using the mask pattern data; computing, by the computer, the product of multiplication of the mutual transmission coefficient and the value obtained by the Fourier transformation; and performing, by the computer, an inverse Fourier transformation on the product.