Point of view: Sales don't equal effectiveness
The purpose of advertising is largely to encourage more consumers to buy your brand more often. Some debate still lingers concerning how this occurs, e.g. attitude shift vs mental availability, but it is uncontroversial that exposure to a brand's advertising should increase the propensity to buy that brand -that's what an advertiser hopes to achieve for their spend.
Yet, this is not what is measured, hardly ever. A bit of this increase in propensity to buy the brand shows up in a change in this week's sales figures. But only a bit, because most of the consumers exposed to the advertising didn't buy from the category this week. What on earth do this week's sales figures tell us about the total (long-term) effect of this bit of advertising? Not much, because we don't know how much of the total effect is represented by the lift in this week's figures. It's like looking at the tip of an iceberg and trying to work out the size of the iceberg when no one knows the ratio of the bit above the water to the bulk below.