International A Level Biology / Cardiovascular health and risk
Evaluating studies of health risk
Edexcel International A Level Biology (YBI11), chapter 3: Cardiovascular health and risk, specification point 1.16. Free lesson.
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Studies of health risks are judged by how they are designed and checked, not by their headlines. Understanding samples, validity and replication helps you decide how convincing the evidence is.
How cohort and case-control studies differ
Both are observational studies: scientists record what happens rather than assigning people a diet or other exposure. A cohort study follows people forward to see who develops cardiovascular disease (CVD). A case-control study starts with people who have CVD and compares them with similar people who do not. A cohort study starts with a large group without the disease. It can measure who develops CVD and therefore gives a measure of risk. The Framingham Heart Study, running since 1948, is an example. Cohort studies can take years and cost a lot. Some people may drop out. A…
How to judge a study's design
A strong design makes results more trustworthy and more useful for the population being studied. When you criticise a study, explain what each design feature affects, such as validity or repeatability. A large sample is less affected by chance differences from a few unusual people. With thousands of people, one person's result has little effect on the mean. A statistical test can check whether a result is significant. A random, representative sample reflects the wider population. Volunteers from one gym may be fitter than most people, so the result may not apply to everyone. Control…
What valid, repeatable and reproducible mean
These terms describe different qualities of evidence. Use the precise term in an exam, rather than saying only that a study is reliable. Valid means the study measures what it is meant to measure. Controlling other variables, for example by matching groups for age, sex and smoking, helps make a study valid. Repeatable means the same person uses the same method and gets similar results. Measuring one person's blood pressure three times checks repeatability and allows a mean to be calculated. Reproducible means someone else, or another method, gets similar results. For example,…
How a study becomes accepted evidence
A single study needs checks before its results become part of a convincing body of evidence. Each step has a different purpose, and peer review does not prove that the result is correct. Scientists write up the method, data and conclusions in a paper. During peer review, other experts check the method, analysis and whether the conclusions follow from the results. They may request changes or reject the paper. If accepted, the study is published in a scientific journal so others can read its method. In replication, other teams repeat the study using new samples. Similar results show…
How to criticise a sample
Check both how many people took part and who they were. A small or unrepresentative sample can limit how confidently the results apply to the wider population. In a study of salt and CVD, 60 volunteers were all men aged 50 to 60 from one running club. The sample is small, so a difference could be due to chance. It is not representative: it includes only men in one age group, and runners may be fitter than most people. The findings cannot safely be applied to women, other ages or inactive people.