Let’s start with a little tutorial (no, please keep reading, it will be brief) and then we’ll talk about why this week’s economic data changes everything, more or less.

The Bank of Canada sets the benchmark overnight rate (the rate at which banks lend to each other). That in turn affects market interest rates on everything from mortgages through to business loans. At present, the overnight rate is at 1 percent, following three hikes of 25 basis points each last year.

The tutorial is on the ‘output gap’ which is one of the major tools that the Bank of Canada looks at to set monetary policy. Here goes.

The ‘output gap’ refers to the difference between the actual output of the economy and the potential output. Potential output basically refers to the maximum that could be produced if all inputs (like the labour force, technology, capital, factory space and all that) were used to the fullest extent that they can be without triggering inflation.  That last little bit is key: when the bank says ‘potential’ they don’t mean full potential, they mean ‘potential without forcing prices higher’. It is a similar concept to what economists mean when they say ‘full employment’. In that case it does not mean everyone working, it means everyone working that can be working without wages being forced higher.

The Bank of Canada monitors the output gap as best they can, first by estimating what potential output is in any period of time, then estimating how close to potential the economy looks to be. A positive output gap means the economy is operating above potential, and that inflation is a risk. A negative gap means there is excess supply (for example, too many unemployed workers) and that inflation is not a risk, or at the extreme, that deflation is possible.

The Bank of Canada adjusts policy to try to get keep things in balance and the output gap closed – sort of a ‘not too hot, not too cold’ thing. Based on their most recent calculations, their latest estimate (which was contained in last week’s Monetary Policy Review) was that the output gap would close by the middle of 2012.

Everybody still with me? Good. Here’s the thing: as well as looking at the output gap itself, the Bank also looks at a bunch of economic indicators to see how close to capacity the Canadian economy is running. Things like industrial production, the unemployment rate, unfilled manufacturing orders – and inflation.

That last one is probably the most important, and it is the one that seems to be running most out of sync with where the Bank of Canada thought it would be. In the Monetary Policy Report, the Bank said that the overall inflation rate (which they target to be 1 to 3 percent) would peak at 3 percent in the second quarter. This week, we got the March inflation report, and we find out that the inflation rate was 3.3 percent as of March –  which is decisively in the first quarter. Ouch.

So what does this mean? It means something has to change to keep the Canadian economy from overheating. That something is likely to be Canadian interest rates, and when I say ‘change’ I mean ‘go higher’.

If rates do not go higher, then the output gap is at risk of going into positive territory, which means inflation takes off even more. No way is the Bank of Canada going to let that happen.

There are other things to take into account too – the spiky Canadian dollar is an important one – but it does not take away from the big picture.

Big picture? A rate hike by July, and maybe more to come after that. And yes, watch the loonie soar in the meantime.