An argument from analogy can be strong. When the likenesses do not matter for the conclusion, it becomes a fallacy.
Key idea
A weak analogy rests on likenesses that are not relevant to the conclusion, or ignores differences that are.
Why it works
An analogy works only when the shared features are a reason for the shared conclusion. Bikes and cars both have wheels, but wheels have nothing to do with fuel, so the likeness gives no reason to think a bike needs gasoline.
Worked example: Relevant or not?
"My bike and a car both have wheels and take you places. Cars need gasoline, so my bike needs gasoline too."
- What does the conclusion need? A shared way of being powered.
- Wheels and travel are not about power.
Answer: Weak analogy.
Worked example: A strong one
"This puzzle box has the same maker, lock and lid pattern as the last one, which opened by sliding the lid left. So this one probably does too."
- What does the conclusion need? A shared way of opening.
- Maker, lock and pattern are exactly what decide how a box opens.
Answer: Not a fallacy.
Common mistake
Calling every comparison weak. Ask what the conclusion needs, then check whether the two cases share that feature. If they do, and no relevant difference is ignored, the analogy can be strong.