BIOLOGY · Stage 8 · Chapter 6: Nutrition, Health and Movement

Biology 6.8 Building the evidence — Epidemiology and Scientific Studies

5 min read 6 parts Lesson 8 of 12

0%read

Lesson summary

Biology 6.8 Building the evidence — Epidemiology and Scientific Studies — Alhussan International Academy — Biology (Stages 7–9) notes: Explain how scientists use correlation and large studies to build evidence about health, and judge how reliable that evidence is.Early belief — t…

First link
Peer-reviewed
More evidence
Dose effect
Conclusion
Causal link

Section 1 of 6The smoking and lung cancer evidence

Early belief
- tobacco has been smoked and chewed for around 500 years,
  - and for most of that time people thought smoking was a good thing, with some doctors even advising it for nerves.
First link
- in the 1940s the British doctor Sir Richard Doll and other doctors collected lifestyle information about their cancer patients.
- Analysing the data, Doll saw that many patients who died of lung cancer were also smokers.
- He published his findings as a peer-reviewed paper in the British Medical Journal.
Peer-reviewed = other scientists read and evaluate the work before it is published.
More evidence
- after his article, scientists in many countries collected more evidence of the link.

Section 2 of 6What epidemiology shows

Epidemiology = the study of how often diseases occur in different groups of people, and why.
Dose effect
- the more cigarettes smoked per day, the higher the risk and death rate from lung cancer.
  - (about 8 times the non-smoker risk for 1–14 a day, 15 times for 15–24 and 25 times for 25 or more)
Time lag
- lung cancer rates follow smoking levels about 20 years later, so the number of deaths falls only after smoking falls.
Conclusion
- up to 90% of lung cancers are caused by smoking, a causal link, and the remaining 10% occur in people who never smoked.

Section 3 of 6Correlation and causation

Correlation = a relationship in which two variables change together.
Causal link = a relationship in which a change in one variable actually makes the other change.
Warning
- a correlation alone does not prove cause, because a third factor may be responsible.
Evidence needed for a causal link
- a plausible mechanism, repeated results from different studies, a dose effect and a fall in risk when people stop.
  - (tar contains carcinogens)
Changing evidence
- for years the evidence came from analysing patient data, and later experiments showed that chemicals in tobacco smoke actually cause lung cancer.
Genes
- genetic information affects the risk,
- so a heavy smoker may live to old age having inherited protective genes, and non-smokers can still get lung cancer.

Section 4 of 6Designing a reliable study

Large sample
- chance variation matters less.
Representative sample
- people of similar age, sex and background so the groups compare fairly.
Control group
- such as non-smokers, for comparison.
Control other variables
- such as age and diet.
Repeat
- results that others obtain too are more reliable.
Ethics
- scientists cannot deliberately expose people to harm, so human health studies follow people who have already made their own choices.

Section 5 of 6Evaluating data and claims

Questions to ask
- how big was the sample, who paid for the study, was it peer reviewed and was a control used?
Individual cases
- one person does not outweigh data from many thousands of cases.

Stronger evidence	Weaker evidence
Large, representative sample	Few people, chosen by the researcher
Control group included	No comparison group
Repeated and peer reviewed	One study, not checked

Conclusions
- a conclusion should claim only what the data show.

Section 6 of 6Why people still smoke

Reading
- people do not read scientific journals, so they do not see the evidence from thousands of cases.
Choice
- people do not want to accept that something they like increases their risk of a terrible disease.
Industry
- cigarette manufacturers do not want their products linked to millions of deaths a year.

Last updated: 14 October 2026