Last week our discussion took us as far as to a description of the normal or Bell curve. See a depiction above. The numbers at the bottom of the graph refer to “standard deviations” from the norm. We were discussing specifically mileage errors on maps. One standard deviation from the mean (the area between -1 and +1 on our graph above) accounts for roughly 68 percent of all the maps in our hypothetical database. Two standard deviations from the mean (the area between -2 and +2) account for roughly 95 percent of the maps. Three standard deviations account for 99 percent. A normal curve for a phenomenon is taken as evidence of randomness. If there were some reason why the mapmakers of 17 th century England were inclined to make a particular error that reason would show up as some non-normality in this chart. Perhaps the roads between these two places were especially good by the standard of the day, and the ease of travel created a general impression that the cities were...