If M2 is the fuel, then inflation is the actual temperature of the room. Inflation is a general increase in prices and a corresponding fall in the purchasing power of money. For the Federal Reserve, maintaining "price stability" means keeping inflation low and predictable . To measure this, they rely on two primary gauges: the Consumer Price Index (CPI) and the Personal Consumption Expenditures (PCE) Price Index.

While both track inflation, they do so through different lenses. Understanding the nuance between them is the key to understanding why the Fed might act even when the "news" says inflation is fine.

The Consumer Price Index (CPI): The People's Metric

The CPI is the most frequently cited measure of inflation in the media . It is released monthly by the Bureau of Labor Statistics (BLS) and measures the price change of a "fixed basket" of goods and services from the perspective of the urban consumer .

The "Basket of Goods"

Imagine a giant shopping cart filled with everything a typical family buys:

Housing (Rent, owners' equivalent rent)
Transportation (Gasoline, new/used cars, airfare)
Food (Groceries, dining out)
Medical Care
Education and Communication

The BLS tracks the prices of these items in various cities and calculates a weighted average. The weights are determined by how much the average consumer spends on each category. For example, housing has a much larger weight than apparel because people spend a larger portion of their income on rent than on shirts .

The Personal Consumption Expenditures (PCE): The Fed's Favorite

While the public watches the CPI, the Federal Reserve officially targets the PCE Price Index . Since 2012, the Fed has set a 2% inflation target based specifically on the PCE .

Why the preference? The PCE is considered a more comprehensive and flexible measure. While the CPI uses a fixed basket of goods, the PCE formula accounts for "substitution" .

The Substitution Effect: A Practical Example

Imagine the price of beef spikes by 20%.

The CPI Approach: The CPI continues to track the price of beef in its "fixed basket," showing a high inflation reading.
The PCE Approach: The PCE recognizes that when beef gets too expensive, consumers might switch to chicken. The PCE adjusts its "basket" to reflect this change in behavior.

Because it accounts for how people actually spend money in real-time, the PCE is often seen as a more accurate reflection of the "cost of living" and the overall inflationary pressure in the economy .

Comparing the Two Gauges

Feature	Consumer Price Index (CPI)	Personal Consumption Expenditures (PCE)
Source Agency	Bureau of Labor Statistics (BLS)	Bureau of Economic Analysis (BEA)
Perspective	Consumer (what you pay)	Business/Economy (what is sold)
Basket Type	Fixed (updated every few years)	Dynamic (accounts for substitution)
Scope	Urban consumers only	All domestic households + non-profits
Fed Status	Secondary/Informational	Primary Target Metric

"Core" vs. "Headline" Inflation

When you read an inflation report, you will often see two numbers: "Headline Inflation" and "Core Inflation." This distinction is crucial for understanding long-term trends.

Headline Inflation: This is the "all-in" number. It includes everything in the basket, including food and energy prices.
Core Inflation: This version excludes food and energy prices .

Why exclude food and gas? These items are notoriously volatile. A hurricane in the Gulf of Mexico can spike gas prices for a month, or a drought in the Midwest can send corn prices soaring. These "temporary supply and demand imbalances" can make the index difficult to compare on a long-term basis . The Fed focuses on "Core" inflation because it provides a clearer picture of the underlying, long-term inflation trend without the "noise" of volatile commodities .

The 2% Target and the Taylor Rule

The Fed aims for 2% inflation because it believes this level supports its dual mandate of low unemployment and stable prices . To decide how to adjust interest rates based on these inflation readings, economists often use the Taylor Rule.

The Taylor Rule is an econometric model that suggests the Fed should raise interest rates when inflation or GDP growth is higher than desired, and lower them when they are lower than desired . It essentially provides a mathematical formula for the "Economic Thermometer":

If Inflation > 2%, Raise Rates (Cool the economy).
If Inflation < 2%, Lower Rates (Heat the economy).

Why Inflation Matters to You

Inflation is often called the "hidden tax." It quietly erodes the purchasing power of your savings . If you have $1,000 in a savings account earning 1% interest, but inflation is 5%, you are effectively losing 4% of your wealth every year. You still have $1,000, but that money buys 4% fewer groceries and gas than it did the year before.

By tracking CPI and PCE, you can anticipate when the Fed is likely to raise interest rates. When rates go up, the interest you earn on savings might increase, but the cost of your mortgage, car loan, or credit card debt will also rise.