JUST how much some brands are affected by price can be seen in the first graph, which plots the average BrandIndex score of six petrol companies (BP, Texaco, Shell, Esso/ExxonMobil, Total) against the inverse of petrol prices.<br /><br />It shows an extraordinary correlation of 0.91. Every price movement has a predictable effect on the brands’ ratings.<br /><br />Methodologically, this is fascinating, because there are not many instances in which we have daily measures of brand health across a sector which reflects in its prices so quickly and universally the shifts in the price of the raw material.<br /><br />It means we have few other – probably no other – case studies for these effects, certainly none so clear-cut. It also attests to the beauty of BrandIndex, which is here shown to be extremely sensitive to people’s response to brands. Readers will be reassured that my weekly pronouncements in this space are based on robust evidence.<br /><br />YouGov has long demonstrated its superior accuracy in predicting elections, so it’s nice to be able to demonstrate the accuracy of our brand tracking too.<br /><br />The second graph shows four of the brands individually. We see that BP and Shell have tracked very closely, with Shell just edging marginally ahead during the past eighteen months.<br /><br />But the really interesting thing is the contrast in the scores for Esso and ExxonMobil. They are the same company, of course. We only started tracking the American version of the brand in May 2008. In that time, the gap between Esso and its parent brand has widened from 4 to 15 points.<br /><br />This difference, and the difference between them and Shell and BP, shows the other half of the story: brand isn’t just price.<br /><br />Stephan Shakespeare is co-founder and chief innovation officer of YouGov.