A 'theoretical fixed basket' doesn't reflect what people buy
When prices rise, consumers buy cheaper goods rather than higher priced goods.
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Rising prices lead to people substituting certain more expensive goods with cheaper ones. For example, when the price of steak rises, households will often buy more chicken and fish instead. This means that the basket that consumers actually purchase is generally lower than a theoretical fixed basket. Economists refer to this as a substitution effect. The Consumer Price Index in the late 1990s started incorporating the substitution effect using geometric weighting, but it still displays the tendency for inflation to lean towards being overstated.
[P1] The substitution effect means that the theoretical fixed basket used to calculate CPI overestimates how much the average household ends up spending.