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2. Immunisation 2/2 Correlation and Causation

3. Linked?

4. 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.

5. 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 notcausation.

6. Finding Links Ideally scientists • change an _________ variable • measure (resulting) changes in the _________ • and keep all the _________ variables the same.

7. 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?

8. 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?

9. 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 vaccinationgiven 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?

10. 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 believedthere was a link. • Further observations were planned, comparing children who had received the vaccination and those who had not.

11. 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.

12. 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?

13. 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.

14. Can you explain...