Disentangling the two relationships is impossible unless a third variable with the right statistical properties, a so-called instrument, can be found and exploited.

ECONOMIST: Economics focus

By the late sixties, bank lending turned more aggressive in the country and real estate speculation became the third cyclical variable governing economic cycles.

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It could be that schizophrenics are more susceptible to the infection, or some third, as yet unidentified variable may be involved.

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About one-third or more of their pay is variable -- cash bonuses and SARs, tied to individual, business and stock market performance.

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Furthermore, we look at each variable by single, one-third plan, half plan, and full ticket plans.

FORBES: 87 Pearls of Sports Business Insights, Facts, and Humor: A Recap of The 2013 MIT Sports Analtyics Conference