Wednesday, December 19, 2012

CPI and Chain Weighted CPI Explained

Both of the following definitions are from Investopedia:

Definition of 'Consumer Price Index - CPI'

A measure that examines the weighted average of prices of a basket of consumer goods and services, such as transportation, food and medical care. The CPI is calculated by taking price changes for each item in the predetermined basket of goods and averaging them; the goods are weighted according to their importance. Changes in CPI are used to assess price changes associated with the cost of living.

Sometimes referred to as "headline inflation."
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Investopedia explains 'Consumer Price Index - CPI'

The U.S. Bureau of Labor Statistics measures two kinds of CPI statistics: CPI for urban wage earners and clerical workers (CPI-W), and the chained CPI for all urban consumers (C-CPI-U). Of the two types of CPI, the C-CPI-U is a better representation of the general public, because it accounts for about 87% of the population.

CPI is one of the most frequently used statistics for identifying periods of inflation or deflation. This is because large rises in CPI during a short period of time typically denote periods of inflation and large drops in CPI during a short period of time usually mark periods of deflation.

Definition of 'Chain Weighted CPI'

An alternative measurement for the Consumer Price Index (CPI), removing the biases associated with new products, changes in quality and discounted prices. The chain weighted CPI incorporates the average changes in the quantity of goods purchased, along with standard pricing effects. This allows the chain weighted CPI to reflect situations where customers shift the weight of their purchases from one area of spending to another.
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Investopedia explains 'Chain Weighted CPI'

The chain weighted CPI incorporates changes in both the quantities and prices of products. For example, let's examine clothing purchases between two years. Last year you bought a sweater for $40 and two t-shirts at $35 each. This year, two sweaters were purchased at $35 each and one t-shirt for $45.

Standard CPI calculations would produce an inflation level of 13.64%
((1 x 35 + 2 x 45)/ (1 x 40 + 2 x 35)) =1.1364

The chain weighted approach estimates inflation to be 4.55%
((2 x 35 + 1 x 45)/ (1 x 40 + 2 x 35)) =1.0455.

Using the chain weighted approach reveals the impact of a customer purchasing more sweaters than t-shirts.
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