Local air pollution begins at home

Activity 2

Factors correlating with production of highly polluting car exhaust

Many researchers, journalists and politicians will quote statistics and speak of correlations in order to prove a point. A correlation is simply an observed relation between two or more factors.

People often assume that a correlation implies a causal link between factors, that is, that one thing is causing the other. Sometimes this is true: for example, there is a clear correlation between smoking cigarettes and dying of lung cancer years later and there appears to be a causal link between these two factors.

But there are also plenty of correlations that do not have a causal link. For example, consider the fact that most people in car accidents were wearing seats belts. Does wearing a seat belt cause you to have a car accident? You know that this isn't true because of your knowledge of what is likely to cause car accidents and because you know that most people who do not have car accidents also wear seat belts. (However, seat belts do tend to reduce certain types of injury to the occupants in car accidents.)

Sometimes two correlated factors may be linked because a third, hidden, factor causes both of them to happen. In that case, neither of the two measured factors directly influences the other, although they may always occur together.

This activity, using fictitious data, is designed to help you think about correlations, and to help you learn what conclusions it is reasonable to draw from limited data.

  • As part of a research study to find out what causes some cars to produce more highly polluting exhaust then others, 20,018 cars in a city were stopped at random and their exhaust tested. Cars found to be producing exhaust above a recommended limit numbered 645. With their owners' consent, those cars above the limit were examined and their drivers questioned. The researchers noted various features about the cars and their drivers. The information is expressed here as a percentage of all the 645 cars found to have exhaust pollution above the limit.
Feature Percentage of cars
Driver was wearing clothes 100
Car was made overseas 66
Person driving was male 58
Occupants were under 25 years old 63
Car was more than 10 years old 82
Engine had not been tuned in the last 6 months 42
Car used unleaded petrol 21
Engine size was below 1500cc 48
  1. Which of the following statements can be assumed from this study? (Give brief reasons explaining your answers.)

    1. A 100 per cent correlation means that you have found the cause of something.
    2. Unleaded petrol causes about one-fifth of cars to produce unacceptable pollution.
    3. People under 25 don't drive as well as older people so, therefore, their cars produce more pollution.
    4. Cars made overseas are more polluting because of their method of manufacture.
    5. Engine tuning is not effective in reducing exhaust pollution.
    6. About half of all cars with engine sizes less than 1500cc produce above-average exhaust pollution.
    7. If car drivers do not wear clothes this will surely reduce vehicle pollution, even though we may not understand why it should do so.

Here is some information about the other 19,373 cars also tested in that study.

Feature Percentage of cars
Driver was wearing clothes 100
Car was made overseas 75
Person driving was male 54
Occupants were under 25 years old 40
Car was more than 10 years old 58
Engine had not been tuned in the last 6 months 32
Car used unleaded petrol 58
Engine size was below 1500cc 41
  1. What conclusions can you now draw from the data and your own knowledge? Give your reasoning for each conclusion.
  2. What further statistics would be useful in helping to draw conclusions about the correlations with, and possible causes of, highly polluting car exhaust?

Teachers notes

  1. The only conclusions that can be drawn from this study are:

    • All drivers wear clothes.
    • More cars in this study were made overseas than were made locally.
    • Older cars are more likely to produce more pollution.
    • More cars producing excess pollution were driven by people under 25. (This may be because younger people can only afford older cars which are more polluting.)
    • Tuning a car's engine may reduce its pollution level.
    • Cars using unleaded petrol are less likely to produce excess pollution than cars using leaded petrol.
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Posted August 1997.