1. Group 2: Nabilah, Jing Kai, Soon Guan
Experimental Design
Group 2: Nabilah, Jing Kai, Soon Guan

2. Outline Experimental Design Prospective Study vs Retrospective Study
Flaws in Studies
Cause and Effect
Case Study: Smoking and Lung Cancer
Conclusion
Outline
Experimental Design

3. Reading
Salsburg, David (2001). The Lady Tasting Tea: How Statistics Revolutionized Science in the Twentieth Century, W.H. Freeman (Chapter 18, Does smoking cause lung cancer?)

4. Prospective Study
Subjects are selected and monitored over a period of time for development of disease
Problems:
Extrapolation issues
Time consuming
-The study normally only focuses on a particular population and hence cannot be generalized with other parts of the world

5. Retrospective Study Subjects with disease are identified
Investigations for prior conditions associated with disease
Problems:
Rare diseases
Not enough/Difficult to find subjects for the study

6. Flaws in Studies Every study has its flaws Non-random samples
→ Reinforcement of results through consistency over many studies

7. Correlation Does Not Imply Causation!

8. A implies B = Not A implies not B
Cause and Effect:
A implies B = Not A implies not B
Defined by the famous scientist, Bertrand Russell in 1930s

9. Postulates for Causation
Whenever the agent can be cultured, the disease is there
Whenever the disease is not there, the agent can’t be cultured
When the agent is removed, the disease goes away
Robert Koch in the 19th century expanded on Russell’s theory

10. Does Smoking Cause Lung Cancer?
Case Study

11. Overview Prospective & retrospective studies done
Widely reported that smoking causes lung cancer
Prospective: Doctors were interviewed about their smoking habits and followed for 5 years, doctors who smoked more had higher probability of contracting lung cancer
Retrospective: In countries eg, US, Japan, France, 10x as many smokers in the lung cancer patients compared to patients without lung cancer

12. However...

13. Flaws in experimental design Publication bias (Fisher, 1958)
Postulates not met
Flaws in experimental design
Publication bias (Fisher, 1958)
Confounders Eg. Genetics
→ When a patient smokes, he develops lung cancer
When a patient doesn’t have lung cancer, it means he doesn’t smoke
When a patient stops smoking, he is cured of lung cancer
But still difficult to establish a conclusion as there are many other factors affecting lung cancer
→ Opportunistic samples
→ Publicity bias: Results are carefully selected to publish whatever is acceptable to the scientific community and reject that which is not acceptable. Speculation that non-inhalers actually had higher risk of lung cancer than inhalers but this wasn’t published, researchers did not address his claims.
→ Confounders eg. Genetics that affect rates of smoking and rates of lung cancer
Cannot say for sure that smoking causes lung cancer

14. Conclusion Correlation does not imply causation
Not easy to determine cause and effect
Studies are still important as they can still give an idea and provide clues on the possible underlying causes of a disease
There could just be statistical correlation.
SG: The evidence that smoking causes cancer: arduous fight against the the tobacco companies. - Took a lot of lobbying by scientist and statisticians to refute the claims made by the tobacco companies -

15. Thank You!