If that happens again, doctors and drug firms will have to drill down into the data to find other differences--such as whether the drugs cause weight gain or other side effects that might become a problem over the long haul.

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The changes could not be explained by other factors - such as mother's age and socioeconomic status or population effects such as changes in air pollution and influenza epidemics.

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Because each week's recipients were selected at random, the survey, taken as a whole, automatically negated the effects of other temporary influences on spending, such as changes in the stock market or car makers' cheap financing deals.

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Economic change in recent decades has made London less reliant on other parts of the UK, and now cities such as Leeds, or Glasgow, look to the spin-out effects of the overheated London financial sector.

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