The nearest neighbor distance ratio (NNDR) serves as a measure of proximity to outliers in the original data set. By definition, the NNDR is calculated as the ratio of the distance to the nearest record (nearest neighbor) and the second (or 3rd, 5th,...) nearest record. When calculating the NNDR for each synthetic records in the original data set, synthetic data points with NNDR close to 0 are close to the original points in regions of sparse data (e.g., original outliers), and NNDR values around 1 indicate that the synthetic data fall within dense regions of the original data.