What is “the” fiscal multiplier? It depends.

This is from today’s WSJ:

Central bankers elsewhere are strongly indicating that they are preparing to open credit spigots to reflate their economies at a time when fiscal policy is stalled or contracting.

Not just “at a time,” but also “because.”  The multiplier is often described as the increase in GDP resulting from a fiscal stimulus, holding monetary policy constant.  But of course that’s not what we are interested in.  We want to know the increase in GDP that will occur, compared to the likely path if fiscal stimulus does not occur.  And that’s a much trickier proposition.