Contemporary estimates put the death toll from 20,000  to as many as 50,000 of which 2,000 were military and 1,000 loyalist civilians. Some modern research argues that these figures may be too high. Firstly, a list of British soldiers killed, compiled for a fund to aid the families of dead soldiers, listed just 530 names. Secondly, professor Louis Cullen, through an examination of depletion of the population in County Wexford between 1798 and 1820, put the fatalities in that county due to the rebellion at 6,000. Historian Thomas Bartlett therefore argues, "a death toll of 10,000 for the entire island would seem to be in order". Other modern historians believe that the death toll may be even higher than contemporary estimates suggest as the widespread fear of repression among relatives of slain rebels led to mass concealment of casualties. By the centenary of the Rebellion in 1898, conservative Irish nationalists and the Catholic Church would both claim that the United Irishmen had been fighting for "Faith and Fatherland", and this version of events is still, to some extent, the lasting popular memory of the rebellion. A series of popular "98 Clubs" were formed. At the bicentenary in 1998, the non-sectarian and democratic ideals of the Rebellion were emphasised in official commemorations, reflecting the desire for reconciliation at the time of the Good Friday Agreement which was hoped would end "The Troubles" in Northern Ireland. The popular Irish ballad The Rising of the Moon recounts the loss of the United Irishmen. Written as early as the 1860s, the song has been since performed by many musicians, including The Dubliners.
Question: How many years did professor Louis Cullen examine the depletion of the population in the County Wexford?
Answer: 22