While the CPI is computed using weighted average of price changes of the selected products (481 products as of 2010), the SPI is by price changes of particular products purchased by individual households. For example, in case of heating, the CPI covers city gas, district heating, kerosene, etc., but individual households would only use one of these heating types, so subjective heating cost may be different. Also, the SPI can vary depending on frequency of purchase, time point of comparison, psychological factors more sensitive to price increase, etc.
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