How Relief, ReliefF and SURF select neighbors. Each panel in this figure shows the genotypes at two markers for a dataset of cases and controls. For the purpose of this example only these two markers will be considered and both are continuous. When analyzing real data, the process of selecting neighbors is the same, however, but there will be thousands of discrete valued markers (SNPs) each of which would be represented by one of thousands of dimensions. The individual for whom neighbors are being found is shown by the filled red circle. The neighbors that each approach uses for weighting are highlighted in blue. Parts A, B, and C represent how Relief, ReliefF and SURF would select neighbors to be used in weighting. Relief selects the nearest individual of the same class (blue circle) and the nearest individual of the other class (blue cross). ReliefF selects some user specified number of individuals (two in this example) to be used for weighting. SURF, instead of using a fixed number of neighbors, uses all individuals within a distance threshold. The dotted line shows a hypothetical distance threshold.