During World War II, the United States (US) Navy used an optical rangefinder on its ships. It consisted of a bar about 10 feet long with openings at the ends and mirrors that allowed someone peering through a lens in the center to see two images of the target whose distance is sought. Statistical models were being widely used, so it was felt necessary to establish the probability distribution of the error involved in using such a device. At a Navy yard in the US, a new rangefinder was mounted on a tower, and a ship was anchored at a known distance from that tower. Contaminated distributions occur when some of the data are generated by the situation being examined, but some of the data come from an irrelevant situation. Contaminating distributions can increase the variance, change the mean, or even effect the correlation.