Inflation is one of the most important issues in economics.
It influences the interest rate we get on our savings and the rate we pay on our mortgages.
Inflation also affects the level of state pensions and benefits, as well as the price of some train tickets.
What is inflation?
Inflation is the rate of change of prices for goods and services.
There are a number of different measures of inflation in use. The most frequently quoted and most significant ones are the Consumer Prices Index (CPI) and the Retail Prices Index (RPI).
Each looks at the prices of hundreds of things we commonly spend money on, including bread, cinema tickets and pints of beer - and tracks how these prices have changed over time.
One of the key differences between the two main indexes is that RPI includes housing costs such as mortgage interest payments and council tax, whereas CPI does not.
The inflation rates are expressed as percentages. If CPI is 3%, this means that on average, the price of products and services we buy is 3% higher than a year earlier.
Or, in other words, we would need to spend 3% more to buy the same things we bought 12 months ago.
Why is it important?
The data from the CPI and RPI rates are used in many ways by the government and businesses, and play an important role in setting economic policy.
That's because the Bank of England uses it to set interest rates. If the Bank's Monetary Policy Committee thinks inflation will be above 2% in the next 18 months or so, it may increase interest rates to try to subdue it.
Conversely if it thinks inflation is likely to be below 2%, it may cut interest rates.
That's why inflation is a crucial factor in determining the rates banks charge for mortgages and the rates they offer on savings accounts.
It also has a direct impact on some people's incomes.
Anything that is described as index-linked rises in line with inflation.
That includes state benefits, pensions and - in part - some train tickets.
Some companies use the level of inflation to set annual pay rises. Recently, however, due to the effects of the recession, many pay settlements have fallen behind price rises.
How is inflation calculated?
Every month the Office for National Statistics (ONS) collects 120,000 prices of goods and services from a wide range of retailers across the country - including online retailers.
Prices are updated every month and price collectors visit the same retailers each time in order to monitor identical goods and make sure they are comparing like with like.
All these prices are combined using information on average household spending patterns to produce an overall prices index.
It also takes into account how much we spend on different items.
So items are weighted - i.e. given more importance in the inflation indexes - according to how much we spend on them.
We typically spend more on fuel than on postage stamps, for example.
So a large rise in the price of petrol and diesel would affect the overall rate of inflation more, as it has a weight of 4% in the RPI.
Meanwhile a rise in the price of stamps is less likely to affect the overall index, as they have a weighting of 0.1%
This guide was compiled with information from Pam Davies at the ONS.
