By comparing wage trends in categories with lots of immigrants against those in groups with only a few, he derives an estimate of immigration's effect.
Rather than comparing wage growth and inflation - economists' standard gauge of whether living standards are rising - the Conservatives prefer to focus on real household disposable income, which is adjusted for taxes and benefits, and rose in 2012, according to the ONS.
It is better than comparing wage differential, as it is exempt from being affected by various institutional factors.
It will take data from the annual survey of hours and earnings and display information about pay so that people are able to compare wages with others in their sector and region.
"In several places it compares wages and working conditions of workers in areas where small-scale Fairtrade-certified tea and coffee farmers were present with those on large-scale plantations in the same regions," it said in a statement.
But most studies have found that the impact, if any, is very slight.Many studies have compared wages and employment in areas with many immigrants to wages and employment in areas with few.
Wages and salaries for civilian workers were up 1.5% before adjusting for inflation in the 12 months ended in September, according to the Labor Department's comprehensive Employment Cost Index, which compares wages in the same jobs and doesn't reflect wages of people switching careers.
