Experimental Design

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

2. 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

5. Retrospective Study • Subjects with disease are identified • Investigations for prior conditions associated with disease Problems: • Rare diseases

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. Cause and Effect: A implies B = Not A implies not B

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

10. Does Smoking Cause Lung Cancer? Case Study

11. Overview • Prospective & retrospective studies done • Widely reported that smoking causes lung cancer

12. However...

13. Postulates not met • Flaws in experimental design • Publication bias (Fisher, 1958) • Confounders Eg. Genetics

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

15. Thank You!