how is the cpi calculated 
the bureau of labor statistics samples 80 000 prices monthly to calculate the cpi weighing the index for each product or service in proportion to its share of recent consumer spending to calculate the overall change in prices the calculation also factors in the substitution effect as consumers shift spending away from the products rising in price on a relative basis the cpi also adjusts for changes in product quality and features the numbers are provided with and without seasonal adjustments 21