Processing and presenting supplied quantitative data

Overview

Processing supplied data actually, index numbers and pie chart good for and where does it fail.

What you need to know

Answer each one before you reveal it.

Questions from this subtopic

Each one with the model answer underneath.

1. 1

   What does processing supplied data actually mean, and why is it worth the time?

   Model answer

   It means doing something to the raw figures so they answer the question - converting counts to percentages or rates, calculating change, working out a cost per unit of benefit, ranking options, or summarising with an average and a measure of spread. It is worth it because processed figures compare options directly, whereas raw figures usually cannot be compared at all, and it demonstrates a skill that simply quoting a number does not.

2. 2

   When should you convert figures to percentages, and how do you calculate a percentage change?

   Model answer

   Convert when you need to compare places, groups or options of different sizes, or to express a part against a whole. Percentage change is the difference between the new and original values, divided by the original value, multiplied by 100: a rise from 250 to 320 properties at risk is (320 minus 250) divided by 250, times 100, which is a 28 per cent increase. Always state the base you have used.

3. 3

   When is a rate the right way to present a supplied figure?

   Model answer

   When the underlying populations differ in size, so a count on its own says more about size than about the issue - deaths, crimes, patients, cars or households are conventionally expressed per 1,000 people, per capita, or per household. Rates put unequal places on a common footing, which is exactly what you need when the options affect areas of different population.

4. 4

   Why is a ratio such as cost per property protected or cost per job created so useful when comparing options?

   Model answer

   Because it collapses two dimensions into one comparable figure and exposes value for money directly. A scheme costing 12 million pounds protecting 400 properties works out at 30,000 pounds per property, against 45,000 for a rival costing 9 million and protecting 200. Show the working, and then say what the ratio does not capture - the number protected may matter less than who they are and what they would lose.

5. 5

   How does ranking options against criteria help, and what is its weakness?

   Model answer

   Ranking each option on each criterion - cost, number benefiting, environmental impact, deliverability - makes the comparison systematic and shows at a glance where an option is consistently strong. Its weakness is that ranks lose magnitude, so an option beaten narrowly on every criterion looks as poor as one beaten heavily, and any weighting you apply to the criteria is a value judgement that you should state rather than hide.

6. 6

   What are index numbers, and when would you use them on supplied data?

   Model answer

   An index sets one value - usually a base year - to 100 and expresses every other value relative to it, so a figure of 118 means 18 per cent above the base. Use them to compare the trends of series measured in different units, such as house prices, visitor numbers and employment, on a single graph. Always name the base year, because the choice of base changes the picture.

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