Paul Johnson of the IFS has written that under Labour “national debt [could be] around £170 billion higher (in today’s terms) by the end of the 2020s than would be achieved through a balanced budget.” That was all that certain newspapers needed to start talking about a borrowing bombshell under Labour.
£170 billion is a meaningless number, and the end of the 2020s is a meaningless date. First, we should put everything as shares of GDP. £170 billion is about 10% of GDP, and debt is currently around 80% of GDP. However it would be completely wrong to infer that under Labour debt to GDP would be 90% of GDP by 2030. If they achieved current balance by financial year 2017/18, then my excel spreadsheet says that with nominal GDP growth of 4% a year, by 2030 debt to GDP would be around 65% of GDP. (A few points below 65% if investment remained at 1.5% of GDP, a few points above it became 2% of GDP.) If the Conservatives balanced the overall deficit each year debt to GDP would be about 47% of GDP by 2030.
So a £170 billion bombshell actually means debt to GDP would have been reduced from 80% of GDP to around 65% of GDP. So the correct headline should have been “debt to GDP cut by a fifth in 2030 under Labour’s plans”. That is debt, which is much more difficult to reduce than the deficit. To say this is a ‘different interpretation’ is too polite – newspaper reports got it completely wrong. Who should you blame for this: Paul Johnson, innumerate journalists, biased newspapers? I’ll leave that to you.
There remains a real question of how quickly debt to GDP should be reduced. In terms of the analysis I did here and here, Labour’s plans - if it did achieve current balance by 2017/18 - are tougher than the path I described as ‘fast’ debt reduction, although not nearly as tough as Osborne’s plans. (This analysis was done before the Autumn Statement, but to pretend that the analysis needs to be revised on that account gives these numbers spurious precision.) However my ‘fast’ path did not keep to current balance after 2020, but had some further deficit reduction over the next five years. (As a result, debt to GDP was below 60% by 2030 under this fast path.) I have not seen Labour commit to sticking to current balance until the end of the 2020s. So in that sense as well the £170 billion number is meaningless.
What you should conclude from this is simple. First, as Paul Johnson and many others have pointed out, both Labour and Conservatives are aiming for tight fiscal policies (tighter than I and others think sensible given the macroeconomic situation), but the Conservatives’ plans involve substantially more cuts than Labour. Both involve reducing debt to GDP quite rapidly, so there is no question that both plans would not trouble the markets. So the only reason for going for Osborne’s plan, now apparently involving budget surpluses, is if you expect another financial crisis in the 2030s, and want debt to GDP to be something like it was before the last one.  Or, as a headline writer might put it (but somehow I doubt many would): “budget surpluses and austerity so we can afford to bail out the banks again soon”.
 For those who are really into fiscal rules, there is a technical question about whether it is better to have a target for the overall deficit or the current balance. As George Osborne has moved from the latter to the former, it may be best to read his detailed analysis of the issue. Cannot find anything? Well maybe, as I note here, he is simply following the discussion in Portes and Wren-Lewis (2014), which argues for deficit targets but a separate target for the public investment to GDP ratio.