This approach has been independently proven to be a leading-indicator of behavior and, axiomatically, since more positive consumer behavior should produce more returns on efforts, can provide marketers with a realproxy for ROI.
Using the Goldman Sachs Commodities Index as a proxy for the costs of mining (because it is heavily weighted towards industrial commodities), the real cost of mining gold jumped by nearly 50% during this period, explaining why gold stocks did not provide the leverage that gold investors expected.