Two mechanics

About this video

Ali's old car is making a grinding noise. Mrs Lund, a mechanic, listens and says it is the brakes: new brakes cost a lot, but she can fit them today. For a second opinion Ali goes to another mechanic, Mr Daud, who says it is a wheel bearing, which is cheaper. Ali knows nothing about cars, and both are experts. He can't check the noise himself. So how can he decide whom to believe?

An expert's word is a good reason to believe, inside their field, and when most experts agree that is strong evidence. But when experts disagree, a non-expert still has things he can judge: not the noise, but the experts. First, reasons he can follow. Mr Daud lifts the car and lets Ali feel the wheel wobble; Mrs Lund points at the brake pads, and they look thick and new. An expert who can show you a reason gives you more than one who only says "trust me". Second, who gains. Mrs Lund would earn more from the expensive job. That doesn't make her wrong, but it is a reason to ask for more than her word. Third, track record: has either been right before? Fourth, a second opinion from a third mechanic who doesn't know the other two is an independent source, and if she agrees with one of them that counts for a lot.

Sometimes the experts in a field honestly disagree, because the question is still open. Then the right answer for a non-expert is not to pick a side but to believe less firmly: the experts aren't sure yet, so I'm not either. This video goes with the When Experts Disagree lesson of Knowledge Level 2 on Knaph.

Transcript

1. 0:00Ali's car grinds. One mechanic says brakes. Another says the wheel bearing.
2. 0:05Ali can't judge the noise. But he can ask each to show a reason.
3. 0:11Mrs Lund earns more from the big job. Not wrong, but ask for more.
4. 0:17Who was right before? And a third mechanic, who knows neither, agrees with one.
5. 0:23If the experts are truly split, don't pick a side. Believe less firmly.
6. 0:29You can judge experts without being one.