bpp.mit.edu
indexes are designed to provide real-time information on major inflation trends, not to forecast official inflation announcements. We are constantly adding new categories of goods, but we do not cover 100% of CPI goods and services. The price of services, in particular, are not easy to find online and therefore are not included in our statistics.
Methodology
Data collection: our data are collected every day from online retailers using a software that scans the underlying code in public webpages and stores the relevant price information in a database. The resulting dataset contains daily prices on the full array of products sold by these retailers. Our data include information on product descriptions, package sizes, brands, special characteristics (e.g. “organic”), and whether the item is on sale or price control.
Daily Online Price Index Computation: The daily online index is an average of individual price changes across multiple categories and retailers. The index uses a basket of goods that changes over time as products appear and disappear from a retailer’s webpage. It is updated on a daily basis and leveraged to estimate annual and monthly inflation. This index is not designed to forecast official inflation announcements, but to provide real-time information on major inflation trends.
Monthly Inflation: The monthly inflation rate is the percentage change between the average of the daily online price index of the last 30 days and the average of the previous month. For example, on the last day of September 2010, we compared the average of the daily index between September 1st and September 30th to the average of the daily index between August 1st and August 31st. On the last day of each month, the value of our monthly inflation is equivalent to the monthly statistic reported by official offices.
Annual Inflation: The annual inflation rate is the percentage change between the average of the daily online price index of the last 30 days and the average for the same period a year ago. For example, on the last day of September 2010, we compare the average of the daily index between September 1st and September 30th 2010 to the average of the daily index between September 1st and September 30th 2009. On the last day of each month, the value of our annual inflation is equivalent to the annual (year-to-year) statistic reported by official offices.
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