The CPI is a fixed-quantity price index, meaning that each month the Bureau of Labor Statistics samples only prices rather than relative quantities purchased by consumers. The problem is that when relative prices of particular goods go up, consumers substitute in favor of other relatively less expensive items. When relative prices go down, consumers do the opposite. An important way that consumers deal with inflation is by buying less of products that become “too expensive.” The result is that the rate of inflation is overstated. Would there be a similar problem during a period of deflation?