The target rating mistake: numerator by target, denominator by everyone

A target rating (TRP) is a share: viewers from the target audience over the size of that same audience. Break the second half — numerator by target, denominator at total population — and you get neither TRP nor GRP. You get a ratio with a mixed base, and a mixed base has no definition.

We have pulled apart enough of these discrepancies to know the pattern. The report sums correctly, the percentages add up, and it still disagrees with the reference table. The data is almost never at fault, nor the measurement method. The culprit is one line in the calculation layer.

Why a mixed base breaks the metric itself

A rating is a proportion, and proportions carry one hard rule: whoever enters the numerator defines the denominator. Pick your viewers as "women 25–54," divide by a universe of 2,295,613 people, and the result compares to nothing. Not the target plan, not total reach, not last quarter's campaign. Nowhere else is that base reproduced.

The danger is how ordinary it looks. The figure lands in a plausible range and computes the same way month after month. Nothing flags. It surfaces only when someone reconciles against an outside source, and that happens late.

How large is the error? Affinity decides

The gap is not random. The relationship is direct:

Affinity measures how much more densely the target audience watches a placement than the population average does. At affinity 200 the target watches twice as densely, and a swapped denominator moves the result by exactly a factor of two.

That is bigger than it sounds. This is not a couple of percent you can write off as rounding or a data-version difference. It is a multiple, and it almost always lands on the flattering side. Better targeting means higher affinity, which means a prettier plan. An analyst carrying this bug gets rewarded most for their sharpest targeting work.

The second way in: averaging days into weeks

The same defect arrives from another direction when daily figures roll up into weekly ones. Take the arithmetic mean of daily percentages and the base shifts silently: every day carries its own, while the week needs a single one. Averaging percentages looks harmless. It is averaging fractions with different denominators, and the result again describes no specific set of people.

So in TV Budgeting and TV Planner we weight multi-day aggregation by universe and compute Reach & Frequency over individuals, not shares. You cannot assemble reach from percentages at all: to count how many different people saw the campaign, you have to work with the people themselves.

How we tested it: cell-level reconciliation

We checked it the blunt way: reproduce the official audience table cell by cell.

• 60,858 cells in the table
• 21,548 non-zero cells, all matched to within 0.5 of a person
• 17 of 17 demographic breaks

What matters there is not the neatness of the match but what it proves. Every break can agree at once only when numerator and denominator describe the same people in each of them. One swapped universe breaks it: any break whose affinity sits meaningfully away from 100 diverges on the spot. Cell-level reconciliation is a consistency test on the base, wearing the costume of an accuracy test.

We reproduce Nielsen's numbers; we do not reinterpret them. The reference table is the arbiter, and our job is to land on it. Nielsen publishes the audience definitions the industry plans against; the Media Rating Council maintains the accreditation standards those measurement services are held to.

A one-minute test

Ask for the base size printed beside every percentage. If the target-audience table and the total-population table show the same base, you have a hybrid on your hands, and there is nothing to compare until the calculation is rebuilt. Adjacent checks — discount ladders, prime splits, durations, post-buy — sit in the buying-software checklist.

In target ratings the error rarely sits in the measurement. It sits in the base. A labeled universe beside every percentage costs less than any after-the-fact reconciliation, and it settles most arguments about whose report is right.