Reasons & Evidence
Understand how reasons and evidence function in arguments and how to distinguish strong from weak support.
Key Terms
Types of Reasons
Reasons can take different forms. Factual reasons present information claimed to be true ("75% of students who sleep 8 hours score higher"). Value judgements express moral or ethical positions ("Education is a fundamental human right"). Causal claims assert that one thing produces another ("Social media causes anxiety in teenagers").
The strength of a reason depends on how directly it supports the conclusion and whether it is true.
A 2023 study of 10,000 patients found that daily exercise reduces the risk of heart disease by 35%. Therefore, the government should promote active lifestyles.
Analysis: The statistical evidence provides a factual reason directly relevant to the conclusion. It is stronger than anecdotal evidence because it is systematic.
My grandfather smoked for 50 years and lived to 90. Clearly, the link between smoking and cancer is overstated.
Analysis: One personal example is anecdotal evidence. It cannot override large-scale statistical evidence and is too small a sample to draw reliable conclusions.
Evaluating Evidence Quality
Not all evidence is equal. When evaluating evidence, consider: sample size (larger = more reliable), methodology (how was data collected?), recency (is the data current?), relevance (does it directly address the claim?), and source (is the source credible and unbiased?).
A common flaw is treating correlation as causation — just because two things happen together does not mean one causes the other.
Countries with higher chocolate consumption have more Nobel Prize winners per capita. Therefore, eating chocolate increases intelligence.
Analysis: This is a correlation, not a causation. Both may be explained by a third factor (wealth/prosperity) rather than chocolate causing intelligence.
Relevance of Reasons
A reason may be true but still fail to adequately support the conclusion if it is not sufficiently relevant. Irrelevant reasons are a form of weak argumentation. When assessing relevance, ask: "If this reason is true, does it actually make the conclusion more likely to be true?"
We should not trust politicians. After all, they wear expensive suits.
Analysis: Even if politicians do wear expensive suits, this has no relevance to whether they are trustworthy. The reason does not support the conclusion.
Cambridge Exam Tips
- 💡When asked to "evaluate the support given for the conclusion," consider both the truth of the reasons AND how directly they connect to the conclusion.
- 💡Distinguish between correlation and causation in statistical evidence — this is a common exam challenge.
- 💡Anecdotal evidence is weaker than systematic evidence but is still a reason — explain WHY it is weak, not just that it is anecdotal.
- 💡Always state what assumption is needed to link a reason to the conclusion when the link is not explicit.
Practice Questions
All practice →Which of the following represents the strongest evidence for the claim that "regular exercise improves mental health"?
Answer in Practice →Identify and explain the flaw in the reasoning about evidence in this argument.
Answer in Practice →