MPs cited that the Office of Budget Responsibility had initially expected 4.5 million people to be on Universal Credit by March 2015, but this has now been down-graded to 400,000. Duncan Smith explained that the figures have changed to ensure the system doesn't buckle under pressure.
"The earlier plan was we were loading large numbers much earlier on and then the system didn't cope.
"Following the rest [of the project], we wanted to do it in a different way, which is where we have the system set up, then bring on the larger numbers towards the later stages because you understand how the system will work under that stress," he said.
"Go slow, test the system, then do the volume."
MPs on the committee told the DWP representatives that they had been made aware of conflicts occurring between the department and the Cabinet Office, after the Government Digital Service (which falls under the Cabinet Office's responsibility) had been brought in to put Universal Credit back on track.
But Shiplee said: "I can't comment on tittle tattle, one has disagreements and one has to get on with things.
"I'm talking to the Cabinet Office about helping us to move things forward."
Shiplee was then asked how sure he was that Universal Credit could be delivered at the scale required for a national rollout, on a scale of one to 10.
"I've never been keen on one to 10s, but I believe Universal Credit can be delivered as it has been delivered. I have no doubt in my mind. It has to be," he said.
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