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Data and Market Analysis

Index numbers and comparing figures over time

  • 3 things to know
  • Component 2

Overview

Setting a base period at 100, converting figures to the index and back, rebasing a series, and the trap of reading a rise from 120 to 150 as thirty per cent.

What you need to know

Answer each one before you reveal it.

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Key term

What is an index number, and how is one calculated?

Key term

What is an index number?

Numbers

State the formula for converting a figure into an index number.

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 figure as a proportion of its value in a chosen base period. The base period is set at 100 and later periods are measured against it.

  2. 2

    State the formula for converting a figure into an index number.

    Model answer

    Index number equals the figure for the period divided by the base-period figure, multiplied by 100. The base period itself always comes out at 100.

  3. 3

    Sales were £4.5m in the base year and £5.4m three years later. Calculate the sales index for the later year.

    Model answer

    £5.4m divided by £4.5m gives 1.2. Multiplied by 100, the index for the later year is 120, so sales are 20% above the base year.

  4. 4

    A price index stands at 128. What does that figure tell a manager?

    Model answer

    Prices are 28% higher than in the base period, when the index stood at 100. The figure says nothing about how prices moved between those two dates.

  5. 5

    Output in the base year was 800 units and the output index now stands at 135. Calculate current output.

    Model answer

    Output equals 800 units multiplied by 135 and then divided by 100. Current output is therefore 1,080 units, a rise of 280 units.

  6. 6

    Why do businesses use index numbers rather than raw figures when comparing several markets?

    Model answer

    An index puts series of very different size onto one common scale starting at 100. Growth rates can then be compared directly on a single graph.

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