Stock
A quantity measured at a specific date, like the water level in a tank.
Not to be confused — It is not a quantity accumulated over a period — that is a flow.
Flow
A quantity measured over a period, like the rate at which a tap runs.
Not to be confused — A flow starts again from zero each period. It cannot be compared directly with a stock, and adding up flows does not necessarily reconstruct the change in a stock.
Percentage point
The difference between two values already expressed as percentages.
A rate that goes from 39.0% to 42.8% gains 3.8 percentage points. The same change is a relative increase of about 9.7%, since 42.8 divided by 39.0 equals 1.097. Both figures are correct and do not measure the same thing.
Not to be confused — This difference is never written “+3.8%”: that would be a relative increase, two and a half times smaller here.
Basis point
One hundredth of a percentage point.
One hundred basis points make one percentage point. A rate that goes from 3.50% to 3.75% gains 0.25 percentage point, i.e. 25 basis points. The unit is used for interest rates, where a tenth of a point is too coarse.
Index point
The unit in which an index is counted, such as a price or production index based at 100.
An index that goes from 7,000 to 7,070 gains 70 index points, i.e. 1%. The same 70 points from 3,500 would be worth 2%: the conversion depends on the starting level.
Not to be confused — An index point is not a percentage and cannot be converted without the level.
Current euros
Amounts expressed at the prices of the year in which they were measured, without adjusting for inflation.
Not to be confused — Two amounts in current euros several decades apart cannot be compared directly: part of the gap is simply the rise in prices.
Constant euros
Amounts brought back to the prices of a single reference year, to neutralise inflation.
Also called “in volume terms” or “in real euros”. The reference year must always be stated: an amount in constant euros means nothing without it.
Ratio
The result of dividing one quantity by another, such as debt relative to GDP.
A ratio moves for three reasons: its numerator changes, its denominator changes, or both change at different rates. Observing its change therefore does not say which of the two quantities moved.
Not to be confused — A ratio is not an amount, and its fall does not imply a fall in what is counted in the numerator.