Smart answer:

Your search for Who said half of advertising money is wasted returned the following results:

The famous John Wanamaker

The famous John Wanamaker was quoted as saying 'Half the money I spend on advertising is wasted, the trouble is I don't know which half.'

Source:

Marketing pioneer John Wanamaker

Marketing pioneer John Wanamaker is attributed as saying, 'Half the money I spend on advertising is wasted; the trouble is I don't know which half.'

Source:

The oft-quoted John Wanamaker

The oft-quoted John Wanamaker once coined the phrase “half the money I spend on advertising is wasted; the trouble is I don’t know which half.”

the American retail pioneer John Wanamaker

More than a century ago the American retail pioneer John Wanamaker was quoted as saying ‘Half the money I spend on advertising is wasted; the trouble is I don’t know which half,’ and the same is true today.

Source:

John Wanamaker, the department store magnate

John Wanamaker, the department store magnate once said, “Half the money I spend on advertising is wasted; the trouble is I don’t know which half.”

Source:

Nineteenth century retailer John Wanamaker

Nineteenth century retailer John Wanamaker is credited with saying “Half the money I spend on advertising is wasted; the trouble is I don't know which half.”

American retail maganate John Wanamaker

As American retail maganate John Wanamaker was quoted as saying – “Half the money I spend on advertising is wasted; the trouble is I don't know which half”.

Source:

Turn of the century merchant John Wanamaker

Turn of the century merchant John Wanamaker is credited with saying, “Half the money I spend on advertising is wasted; the trouble is I don’t know which half.”

Pioneering Philadelphia retailer John Wanamaker

Pioneering Philadelphia retailer John Wanamaker is often quoted as saying, “Half the money I spend on advertising is wasted; the trouble is, I don’t know which half.”

Department store merchant John Wanamaker

Department store merchant John Wanamaker is famously quoted as saying, “Half the money I spend on advertising is wasted; the trouble is, I don’t know which half.”