Why it matters
Purchasing power is the translation step between economic statistics and lived experience, and it is the step most reporting skips. Nominal figures are the ones that get published: revenue up, wages up, the index at a record. Whether any of that constitutes an improvement depends entirely on what happened to prices over the same period, and that comparison has to be made deliberately because nothing in the nominal number reveals it.
The gap compounds. Over one year the difference between a nominal and a real figure is usually small enough to ignore; over a decade it is often the entire story. This is why a period can register measurable growth in every published aggregate while the people inside it correctly report that nothing improved — both descriptions can be accurate, because they are measuring different quantities.
It also explains where the burden of inflation falls. Anyone holding a claim fixed in nominal terms — a saver, a pensioner on an unindexed entitlement, a lender — loses purchasing power passively. Anyone owing one gains. No transaction occurs and no decision is taken; the redistribution is a property of the arithmetic.
What to watch for
Money illusion is the default setting. People reliably reason in nominal terms, treating a pay rise as a gain without checking it against prices. This is not innumeracy — nominal figures are the ones presented, and the correction requires an extra step and an external number.
“Real” always means adjusted, and the adjustment is contestable. A real figure depends on which price index was used. A basket weighted for average consumption will understate the experience of a household whose spending is concentrated in the categories that rose fastest. The word carries an air of objectivity that the underlying choice does not entirely support.
Averages hide distribution. Aggregate purchasing power can hold steady while it falls sharply for households spending a large share of income on energy, rent and food — the categories with the least room to substitute.
Per capita is a third question. A real total can rise while the real figure per person falls, if population grows faster. Which of the three is quoted usually depends on which supports the point being made.
National statistical offices publish both nominal and price-adjusted series — real disposable income, real wages — and FRED carries them alongside the deflators used to produce them.