Questions
Strength is whether it can change the claim
Strong evidence is evidence that could have told you something you did not want to hear. That simple test separates most of what persuades people from what should. Below are the qualities that make evidence strong, the shortcuts that only make it look strong, and a way to grade what you have in front of you.
Measured beats remembered
A figure taken from a system, a meter, an invoice or a log is stronger than the same figure recalled in conversation. Memory rounds numbers toward the story the speaker prefers, usually without any intent to mislead.
When someone quotes a figure, ask where it was recorded. If the answer is that everyone knows it, treat it as a claim in need of a source.
Independence and the number of cases
Evidence from someone who gains nothing from the conclusion is stronger than evidence from someone who gains a great deal. A supplier's figure for its own defect rate is weaker than the same figure from your receiving inspection.
Count matters too. One customer's story is an anecdote, vivid and memorable and easily unrepresentative. Forty customers' records describe a pattern. Be careful with averages, though: a mean of forty records can hide a few extreme cases that matter more than the average.
Shortcuts that only look strong
Confident tone, a famous name, a long document, and many sources repeating one original are the usual counterfeits. Each makes evidence feel stronger without adding anything that bears on the claim.
A related trap is the before and after comparison with nothing to compare it to. Sales rose after the new campaign, but they also rose for competitors who ran none. Without a comparison group, the campaign's share of the rise is unknown.
How the Inspector grades it
In a Decision Room, the Scout gathers public sources and the Inspector checks what each one actually proves, marking every claim supported, partly supported, unverified or contradicted. Strong evidence moves a claim to supported. Weak evidence leaves it unverified, however much of it there is. The Verdict rests on those marks, and the Human Decision is made with them in view.
Three kinds of proof for one claim
A team claims a new onboarding flow cut customer cancellations. Evidence one: the product manager remembers complaints falling. Evidence two: cancellations fell from 6 percent to 4.5 percent in the month after launch. Evidence three: half of new customers were randomly given the new flow for eight weeks, and their cancellation rate was 4.3 percent against 5.8 percent for the rest. The first is a recollection. The second is a measurement that a seasonal dip could explain. The third could have shown no difference, and it did not, which makes it the strongest by far.
Questions
- Is expert opinion strong evidence?
- It is strong when the expert has direct knowledge of the question and nothing to gain from the answer. It is weaker when the expert is speaking outside their field or for a party with an interest.
- Can a single piece of evidence be decisive?
- Yes, when it bears directly on the claim and could easily have gone the other way. A signed contract or a failed safety test can outweigh a pile of softer material.
- How should I record weak evidence?
- Keep it and label it honestly. Weak evidence is still a clue about where to look, and recording it as weak keeps it from being treated as proof later.
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