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Globalisation

Index numbers

  • 3 things to know
  • Papers 2 and 3

Overview

Index numbers and the base year of 100, the formula for an index and calculating one, percentage changes against point changes in an index, weighted indices and revising weights, working back to actual values, comparing indices with different base years, values below 100, presenting data as an index, and reading an index over time.

What you need to know

Answer each one before you reveal it.

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Key term

Why is the base year of an index set at 100?

Key term

Give the formula for an index number.

Numbers

Output was 250,000 tonnes in the base year and 320,000 tonnes this year. Calculate the index number for this year.

Questions from this subtopic

Each one with the model answer underneath.

  1. 1

    What is an index number?

    Model answer

    An index number expresses a value as a proportion of its value in a chosen base year, with the base year set equal to 100, so that changes over time can be compared easily.

  2. 2

    Why is the base year of an index set at 100?

    Model answer

    Setting the base at 100 means any later figure can be read directly as a percentage of the base year, so the change since then is visible at a glance.

  3. 3

    Give the formula for an index number.

    Model answer

    Index number = (value in the current year ÷ value in the base year) × 100.

  4. 4

    Output was 250,000 tonnes in the base year and 320,000 tonnes this year. Calculate the index number for this year.

    Model answer

    The index is (320,000 ÷ 250,000) × 100 = 128.

  5. 5

    A price index stands at 138. By what percentage have prices risen since the base year?

    Model answer

    Prices have risen by 38% since the base year.

  6. 6

    An index rises from 125 to 150. Calculate the percentage increase.

    Model answer

    The increase is (150 − 125) ÷ 125 × 100 = 20%.

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