To save power, for instance, the XO's liquid-crystal display (the biggest consumer of juice in a laptop) can be flipped from backlit colour to self-reflecting monochrome.

ECONOMIST: The $100 laptop is fast becoming a reality

In other words, I might answer a survey as if I were an ideal version of myself rather than reflecting a true self-analysis.

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It is not clear, however, that the same people appear in the bottom quintile in 1999 as did in 1977: most studies show a rapid turnover of people in the lowest income group, reflecting temporary problems or self-employed people exploiting tax loopholes.

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