Mr Duncan Smith hailed the figures, saying the cap had provided a "strong incentive" for people to look for jobs, even before it had started to affect their incomes.
"Already we've seen 8,000 people who would have been affected by the cap move into jobs. This clearly demonstrates that the cap is having the desired impact," he told the Daily Mail.
However Jonathan Portes, the National Institute of Economic and Social Research and a former chief economist at the DWP, said there is "no evidence at all" that the cap has affected people's behaviour.
"The actual analysis published by the Department for Work and Pensions makes it quite clear that they do not attempt to analyse any impact of behavioural change and that there is as yet no evidence one way or the other that there is behavioural change," he told the BBC Radio 4 Today programme.
"It may be that the benefit cap has indeed had the effect that Iain Duncan Smith would like it to have. That is perfectly possible, but without doing the analysis - and it has not been done - you simply cannot say that and you shouldn't say it."
Mr Portes claimed it was part of a "consistent pattern" which threatens to undermine public confidence in official statistics.
But the DWP said it had followed the correct procedures in publishing the data.
"The original forecasts were based on impact assessments last year and have now been updated due to further developments," a spokesman said. "We have followed the correct procedures for publishing this data by our statisticians and it is available for anyone to study. Claims to the contrary are utterly unfounded."
Belfast Telegraph
