When sales rise during a campaign, it is tempting to give the campaign the credit. But that is correlation: the two happened at the same time. Causation is how much of the rise the campaign actually produced. Always measure the second, and never report the first as if it were the second.
Lidl is a good example. Its sales in the UK grew by £2.56 billion between 2013 and 2017. In 2014 it went on national TV for the first time, and growth went from 9.5% a year before the ads to 15.5% after. That timing looks like proof, but on its own it is only correlation.
But Lidl's award paper doesn't stop at the timing. The team used a statistical model to split the growth by cause: the ads, the new stores, and everything else. The ads were given about a quarter, £627 million of the £2.56 billion.
From Lidl's award paper, IPA Effectiveness Awards, Gold, 2018. The split comes from Lidl's own statistical model.
By our sum, had Lidl kept growing at 9.5% a year, it would have added about £1.4 billion anyway. So about £1.1 billion sat above the old line. The ads were given £627 million in all, a bit over half of that. So even the growth above the old line wasn't all down to the ads.
One caution. The quarter comes from Lidl's own model, and the paper doesn't show how the model was built. Its headline figure, £2.7 billion over four years, is a different sum: all the sales the ads drove each year, not their share of the growth.
The case won Gold at the IPA Effectiveness Awards in 2018. The full paper has far more and is worth reading.
Sam