The Consumer Price Index (CPI) is used to measure the overall price level. This index plays an important role in the economy because the rate of inflation in the economy is assessed by the percentage changes in the CPI. The United States Bureau of Labor Statistics (BLS) is responsible for calculating the CPI by studying retail prices and changes in the cost of the basket of goods purchased by a "typical" consumer. The BLS publishes the CPI each month. There are a few steps to measuring CPI. First, the base year in which the economy and prices are stable is chosen. Next, the basket of goods consisting of goods and services that a typical household would use is selected. After that, the prices of the selected products are obtained through surveys and finally, each item is given a different weight to determine its importance by recording the amount of money the consumer spends on each item. Each step is important for the IPC results. When there are errors in the procedure, there will be a bias in the CPI, which will not reflect the true cost of living. Lebow and Rudd list several sources of bias. in CPI, such as higher level substitution bias, lower level substitution bias, new outlet bias, weighting bias and quality change and new item bias. Higher-order substitution bias occurs when increases in the real cost of living are overestimated, with the CPI neglecting consumer substitution effects due to changes in relative prices. This bias, which was only 0.1 to 0.2% per year, increased significantly in the late 1990s. Another substitution bias that occurs within item-area strata, and the bias due to the inability to capture this type of substitution, is known as lower-level substitution. The authors argue that this is what happens in the CPI measurement. Even though each source of bias is not large, when each small bias combines and is derived into the CPI, the level of bias increases. It is important that policy makers realize this and put more effort into investigating and recording data. For example, when the BLS conducts a survey on bread consumption, it may ask all members of a household to respond to the survey instead of just letting the head of the household respond. Apart from this, the survey on the price and weight of the goods and services consumed can be carried out more frequently so that the details are up to date. This could improve the understatement or overspend condition. As the CPI is derived from collected data, more precise data can help increase the accuracy of the CPI..