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 real proxy for ROI.

FORBES: A Non-Cynical View of Return On Investment

Google Chief Economist Hal Varian has shown that search can act as a proxy for purchasing intent in markets like autos and real estate.

FORBES: Eduardo Saverin Jeopardy for Zuckerberg as Oscars Beckon for The Social Network

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.

FORBES: Gold Mining Stocks Are An Increasingly Attractive Opportunity