# Good reasoning strategies

## Causal Reasoning

A and B occur together.

A occurs before B.

Changing A changes B.

A is a better explanation of B than other possible causes.

Therefore, A causes B.

Examples: sex causes pregnancy, HIV causes AIDS.

## Correlation

A and B tend to occur together.

Not-A and not-B tend to occur together.

A and not-B do not tend to occur together.

Not-A and B do not tend to occur together.

Therefore, A and B are positively correlated.

Consider all four cells:

 B Not B A don't just focus on this cell look for counterexamples Not A

## Inference to the best explanation

The competing hypotheses are H1, H2 ...

The available evidence is E1, E2, ...

Hypothesis H1 is a better explanation of the evidence than the other hypotheses, because:

• It explains more evidence.
• It is simpler, i.e. requires fewer extra assumptions.
• It fits better with the rest of what we believe.

Therefore, we are justified in believing H1.

## How to evaluate a medical claim

1. What is the claim?

2. Who is making the claim?

3. What is the evidence?

4. What is the best explanation of the evidence?

Updated Jan. 27,, 2003

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