"When propane costs go up, CPI-U and CPI-W include that as an increase in the cost of living. But some people would just stop using propane if its price went up. They'd switch to electric heating, or a geothermal system, or a wood stove. So their actual heating costs wouldn't go up as much as CPI-U and CPI-W would suggest. Chained CPI attempts to take 'substitution effects' like this into account. Thus, its number generally rises more slowly than other metrics. That adds up to a big cut in Social Security benefits."

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