The Consumer Price Index is a measure of the change over time in the average prices paid by consumers for a market basket of consumer goods and services. As the most widely used measure of inflation, changes in the index are closely followed by policymakers, financial markets, businesses, and consumers. The Office for National Statistics reports both an ‘All Items’ number that includes all categories in the calculation basket, and a ‘Core’ number that strips out highly volatile food, energy, alcohol, and tobacco prices.