A question I don’t expect my colleague Julie Etchingham to put to the two party leaders in ITV’s debate is the one that has been nagging away at me for days: why is the policy-making that will underpin the election manifestos we’ll get from the parties over the next few days so lacking in intellectual rigour?

What we have from both parties are big ambitions – ‘free broadband for everyone’ from Labour, ‘Brexit totally done and dusted by the end of 2020’ from Boris Johnson – based either on a highly selective use of second-hand research (Labour’s analysis of the costs...