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It’s hard to keep on top of this file: there are a lot of numbers to keep track of, and a new set of projections is published every six months. But there is one number that has stayed the same since at least 2014, and it’s probably the most important one to keep in mind: federal government revenues fluctuate in a narrow range around 14.5 per cent of GDP. This ratio — the lowest it’s been since before the Second World War — is the enduring fiscal legacy of the Conservative government.

If there are no changes in the tax structure — and the Liberals have so far not made or promised significant changes — then government revenues will grow roughly in line with the economy, which means that the ratio of revenues to GDP will remain roughly constant. In both the 2014 (the last long-term projection under the Conservatives) and the most recent exercise under the Liberals, projected federal revenues were assumed to be 14.5 per cent of projected GDP throughout the forecast horizon. Put another way, government revenue projections are a combination of economics and politics: economics provides projections for GDP, and the share of GDP that goes to the government is a political decision.

This logic works on the other side as well. For a given set of publicly-provided services — and again, this is a political decision — we’d expect the level of spending required to sustain them to increase with GDP as well. (This assumes that costs increase with the population and with inflation, that productivity growth is hard to wrest from the public sector, and that public servants will demand the same increases in real incomes that other workers receive.)