Thursday + Gregory Huffstutter = The Ad Man Answers
Q: I googled a search on the direct relationship between Impressions and GRPs (Gross Rating Points) and one of your articles came up. I work in advertising and a client brought up the question, do GRPs and Impressions ALWAYS go in the same direction? If ratings increase, should Impressions as well?
A: In a word… Yes.
GRPs and Impressions are directly related to each other, and should always go in the same direction. As your advertising Impressions increase, so will your Gross Rating Points, and vice versa.
The one caveat is that you must be talking about the same Universe, be it total US TV households, adults within a specific media market, etc.
Here’s the formula:
(Gross Impressions / Universe) x 100 = GRPs
Think of it this way… if you’ve bought an outdoor billboard on a major Los Angeles freeway it may generate 1,000,000 Impressions per day – or a million daily OPPORTUNITES to see your advertisement.
Note, I specifically did not say a million PEOPLE, because Impressions are a measure of duplicated audience. So a single taxi driver could pass by your outdoor billboard a dozen times in 24 hours, and he’d be counted as a 12 Impressions that same day.
Those 1,000,000 daily Impressions might only represent 300,000 actual people, depending on that freeway’s traffic patterns. And it also doesn’t mean 100% of those adult commuters will look up at the right moment (although that will soon be tracked as well).
That said, each day that passes, you keep adding 1,000,000 advertising impressions, until your single billboard has chalked up 30,000,000 Gross Impressions over a month.
So how does that relate to GRPs, aka Rating Points?
Rating Points measure the percent of total audience reached by a single advertisement, with 1 Rating Point representing 1% of a given population. So in a market like Los Angeles, where we have approximately 12,545,700 adults, 1 Rating Point represents 125,545 adults with the opportunity to see your message.
GRPs (Gross Rating Points) are the total Ratings Points delivered by a schedule of multiple advertisements. Like Impressions, GRPs are a measure of duplicated audience, because individuals are often exposed several times to the same advertising message.
Continuing with our example, and referring back to the formula, if your one freeway billboard has the audience potential of 30,000,000 Gross Impressions per month, and Los Angeles has a Universe of 12,545,700 adults, then you’ve delivered the equivalent of 239 GRPs.
(30,000,000 Gross Impressions / 12,545,700 Adults in LA) x 100 = 239 GRPs
GRPs can also be expressed as Reach % x Frequency. So if you know that over the course of a month, your one freeway billboard has reached 50% of the LA adult population at least once, then commuters have passed your billboard an average of nearly 5 times.
239 GRPs = (50% Reach) x (4.8 Avg. Frequency)
Now what happens if you run your same billboard for two months? You keep generating 1,000,000 daily Impressions until your advertising campaign has now delivered 60,000,000 Gross Impressions. LA’s population base has stayed the same (no need to quibble over Southern California’s immigration policies, thank you), so your two-month advertising campaign has now hit 478 GRPs:
(60,000,000 Gross Impressions / 12,545,700 Adults in LA) x 100 = 478 GRPs
Impressions are up and GRPs are up. But let’s assume you’ll only reach a 5% of new adults in month two of your campaign, because almost everyone who regularly drives that freeway has seen your billboard at least once. So if your reach only increases from 50% to 55%, that means you’ve now achieved an average frequency of 8.7:
478 GRPs = (55% Reach) x (8.7 Avg. Frequency)
Now let’s say you take a few months off before starting a new advertising campaign, but you can no longer afford the freeway billboard. So you buy a poster on El Segundo Boulevard in LA that only generates 25,000 daily Impressions, instead of a million. After a month, you’ve accumulated 750,000 Gross Impressions, which translates to only 6 GRPs:
(750,000 Gross Impressions / 12,545,700 Adults in LA) x 100 = 6 GRPs
Your Impressions are down and GRPs are down from the original freeway billboard campaign. And vice-versa. As you reduce your GRPs, you’re indicating that your Gross Impressions have also gone down.
The only way to change this relationship is by changing your Universe. So let’s say for month two of your follow-up advertising campaign, you switch your El Segundo Boulevard location for a state highway in Medford, Oregon. Because it’s a small town, this highway only generates 5,000 daily Impressions, so for month two, you only achieve 150,000 Gross Impressions. According to Nielsen estimates, Medford only has 171,830 adults aged 18 or older, so using our formula:
(150,000 Gross Impressions / 171,830 Adults in Medford) x 100 = 87 GRPs
In other words, the Gross Impressions have gone down as you’ve moved your billboard location from El Segundo Boulevard to Medford, Oregon. But the GRPs have increased from 6 to 87, because you’re now measuring those Impressions against a smaller population universe.
Whew – my brain hurts!
Gregory Huffstutter has been punching Ad Agency timecards for the past dozen years, working on accounts like McDonald's, KIA Motors, Suzuki Automotive, and the San Diego Padres. His first mystery, KATZ CRADLE is on submission while he's working on the sequel. The first 100 pages of his novel are linked here. For general advertising questions, leave a comment or send e-mail to katz @ gregoryhuffstutter dot com with 'Ask The Ad Man' in the subject line.