I do not understand why it’s hard to see
at least the end result.
The longer it takes for a change in input to produce a change in output, the greater the chance confounding factors will interfere, making your conclusions less convincing.
You moved your spending from billboards to print magazines, and 6 months later sales are higher. How do you tell if it was the ads, when there was also a model refresh, your competitor released a poorly reviewed product, and the economy has been kind to your target demographic?
As John Wanamaker supposedly said, "Half the money I spend on advertising is wasted; the trouble is I don't know which half"
You moved your spending from billboards to print magazines, and 6 months later sales are higher. How do you tell if it was the ads, when there was also a model refresh, your competitor released a poorly reviewed product, and the economy has been kind to your target demographic?
As John Wanamaker supposedly said, "Half the money I spend on advertising is wasted; the trouble is I don't know which half"