Wason’s four card task

One of the necessary traits in being a tester is being a critical thinker. This skill, when applied in testing, can surely help us in identifying the parts of the program that are potentially more buggy than the rest. It can also help us in tracing the causes behind the bugs we encounter. Now here’s a simple exercise on critical thinking:

Given four cards (roughly depicted below) — each having a letter on one side, and a number on the other, but you can only see one side of the card at a time… Which cards need to be turned over to verify or negate the claim that “If a card has a vowel on one side, then it has an even number on the other side.”

A D 4 7

Most people to whom this test is given to usually fail to give the correct answer since they fall into two traps. One is confirmation bias — they attempt to confirm the claim and forget that it’s also important to try and falsify it. The other is our natural tendency to satisfice i.e., to jump at the first adequate and logical solution, rather than the optimal solution. Chances are they go for turning over {A} or {A and 4}.

Turning over {D} or {4} wouldn’t prove or disprove the claim. If you check the claim closely, there’s nothing that limits the kind of number for consonant cards, and there’s also nothing about even-numbered cards having only vowels on the other side. Turning over {A} would be a correct move — albeit partial. If the other side of {A} reveals an odd number, then the claim is immediately negated. BUT, if it has an even number, then the claim holds true so far. To completely confirm the claim, you would also need to check that {7} has a consonant at the other side. Otherwise, i.e., if {7} has a vowel, the claim is falsified.


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