Scotland faces second budget black hole over £3.5bn benefits bill
Holyrood is to take control of a raft of new benefits such as Attendance Allowance, Disability Living Allowance and Personal Independent payments next year.
It comes after the Commission warned last week that Holyrood's devolved budget is facing a £1 billion spending black hole over the next three years over a shortfall income tax levels.
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Hide Ad"Forecasting social security benefits is difficult," said Commission chief executive John Ireland.
"I would expected there to be some significant forecast errors - I can't say which way but it's a pretty uncertain forecast."
Mr Arthur asked whether the Commission could set out the potential "percentage" for errors to provide an idea of the likely impact.
But commission member Professor Alasdair Smith added: "I don't think we're in a position to do that but it's one of the highlights of our report that we think the social security budget of £3.5 billion, with the possibility of eligibility changes coming into that, is a sum of money with a high degree of risk.
"That's one of the highlights of our report - we're drawing attention to the fact that there's a very significant fiscal risk in this."