Net migration is the difference between two flows. It is the number of people entering a place in a particular year, less the number of people entering a place over the course of that year. As Danny Dorling has pointed out: "Immigration rules were tightened in 1968 with the short-term effect that some 104,000 (net) left during the six years of Labour government (1964-70) and despite the mini economic boom. That exodus was reversed at the start of Ted Heath's tenure (in 1971-2), but then it flip-flopped back to a net migration of (-112,000) under Labour from 1973-9, then again at -105,000 (net) in the three Thatcher years of 1980-2 and yet again a figure of -76,000 (net) during John Major's 1990-3 recession. I point all this out in case you thought that usually more people tended to arrive than leave. In most years since 1840 that has not been the case." (22)