The basic technique is simple. We assume our markers are biallelic, for example, biallelic single nucleotide polymorphisms (SNPs). Regard the data as a large rectangular matrix C, with rows indexed by individuals, and columns indexed by polymorphic markers. For each marker choose a reference and variant allele. We suppose we have n such markers and m individuals. Let C(i,j) be the number of variant alleles for marker j, individual i. (Thus for autosomal data we have C(i,j) is 0,1 or 2.) For now suppose that there is no missing data. From each column we subtract the column means. So set for column j: and then the corrected entries are:
