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Inflation and Deflation

Calculating index numbers and the inflation rate

  • 3 things to know
  • All three components

Overview

Formula for a simple price index, rebasing an index and nominal value and a real value.

What you need to know

Answer each one before you reveal it.

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Numbers

State the formula for a simple price index.

Numbers

A good costs £2.00 in the base year and £2.50 now. Calculate the price index.

Numbers

A price index stands at 120 in one year and 126 the next. Calculate the rate of inflation.

Questions from this subtopic

Each one with the model answer underneath.

  1. 1

    State the formula for a simple price index.

    Model answer

    A simple price index equals the current price divided by the base year price, multiplied by 100. The base year index equals 100 by construction.

  2. 2

    A good costs £2.00 in the base year and £2.50 now. Calculate the price index.

    Model answer

    The index is 2.50 ÷ 2.00 × 100, which equals 125. Prices are therefore 25% above the base year.

  3. 3

    A price index stands at 120 in one year and 126 the next. Calculate the rate of inflation.

    Model answer

    The inflation rate is (126 − 120) ÷ 120 × 100, which equals 5%. Prices rose by 5% over that year.

  4. 4

    A basket has food weighted 40, transport weighted 35 and housing weighted 25, with price indices of 110, 104 and 120. Calculate the weighted index.

    Model answer

    The weighted index is (40 × 110 + 35 × 104 + 25 × 120) ÷ 100. The arithmetic gives 11,040 ÷ 100, which equals 110.4.

  5. 5

    A weighted price index rises from 100 to 110.4 over a year. State the rate of inflation.

    Model answer

    The inflation rate is 10.4%, because the index rose 10.4 points from a base of 100. A rise from a base of 100 can be read directly as a percentage.

  6. 6

    What does rebasing an index mean?

    Model answer

    Rebasing means the index is reset to 100 in a chosen new reference year. The index numbers change but the underlying rate of inflation does not.

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