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Correlation and Causation

Immunisation 2/2

Linked?

growing facial hair producing sperm

height shoe size

final exams sunny weather

ice cream sales deaths by drowning at the beach

star sign academic success

Definitions

Correlation When things happen at the same time e.g. when two variables change

simultaneously.

Causation When changes in one variable cause a change in another.

What’s the Difference?

Ice cream sales don’t cause people to drown.

Drowning people don’t cause ice cream sales to go up.

Instead, both variables are affected by a third – how nice the conditions are at the beach.

There is correlation but not causation.

Finding Links

Ideally scientists change an _________ variable measure (resulting) changes in the

_________ and keep all the _________ variables

the same.

Finding Links

Ideally scientists change an independent variable measure (resulting) changes in the

dependent and keep all the control variables the

same.

Why would this be difficult when investigating possible causes of medical conditions in real people?

Mobile Phones and Babies

Data is available for the number of mobile phone masts in an area and the number of babies born in any given year.

Choose an appropriate graph and use the data provided to see if you can find a link.

Can you explain it?

A Suggested Link

In 1998 a UK doctor, Andrew Wakefield, was one of 13 authors of a paper suggesting that there was evidence linking the MMR vaccination given to young children and the development of autism, associated with unusual bowel conditions.

What questions would you want to ask about the findings and methods of the paper?

A Suggested Link

Study was very small – only 12 children.

Soon after publication questions were raised about contamination of the bowel samples.

In several cases children had taken part in the study because their parents already believed there was a link.

Further observations were planned, comparing children who had received the vaccination and those who had not.

Media Attention

Causes for medical conditions and links between symptoms are suggested all the time. We call any suggested link between variables a hypothesis.

The problem was that newspapers and TV stations, first in Britain then worldwide, published the possibility of a link as a fact – without any discussion of other possible explanations, or the limitations of the study.

Predictions

What do you expect happened to: The vaccination rate for MMR? The overall vaccination rate? The number of parents to autistic

children who believed it was linked to MMR?

The number of cases of measles, mumps and rubella?

Advice?

It is 1998 and some parents have suggested cancelling the MMR vaccination programme.

You have been asked to write a short paragraph explaining to them why this would be an over-reaction.

Use what you know about the risks of measles, mumps and rubella.

Can you explain...