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Diminishing returns
The principle that each additional dollar spent on a channel tends to generate less incremental return than the dollar before it.
Example
Doubling out-of-home spend rarely doubles the resulting reach or sales impact, because the most efficient placements are already bought.
Why marketers care
It's the core reason budget shouldn't simply be poured into whichever channel currently has the best ROI — that ROI will fall as spend increases.