"People who choose to eat more fish are more likely to eat heart healthier diets and engage in more physical activity, " and studies testing the benefit of supplements may not be able to completely adjust for differences like these, said Alice Lichtenstein, director of the cardiovascular nutrition lab at Tufts University in Boston.

NPR: Study: Fish Oil Doesn't Help Prevent Heart Attacks

Two other gauges do adjust for quality differences, mainly by dealing only with repeat home sales.

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Second, adjust the statistical results downward to account for differences between test conditions and the real world.

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