Logic Errors in Testing

While testing it’s easy to fall foul of logic errors in your thought processes. One of the most common errors is the post hoc fallacy.

This is a logic error or fallacy of post hoc, ergo propter hoc (after this, therefore because of this). It can sometimes be all too easy to attribute issues to unrelated events just because they follow them.

For example, a new tenant moves into a new flat and upon arrival the flats hot water boiler goes faulty. The landlord automatically assumes the arrival of the new tenant caused the fault. This is an example of the post hoc fallacy.

In testing we must resist the urge to jump to conclusions, and instead use investigation, logic and critical thinking.

2 thoughts on “Logic Errors in Testing”

  1. But bear in mind that even after you’ve applied investigation, logic and critical thinking, it may still be that the root cause of the fault is the event. The new tenant may have applied different settings to the boiler; or they may have moved in at the end of a hot summer where the boiler hasn’t been used, and so they are first to re-light it after anything up to four months of disuse. In this case, the new tenant is the cause, the change in the use scenario that precipitates the fault condition.

    Only where the boiler fails coincidentally after the tenancy changes hands, where the new tenant has done nothing different to the old but then gets blamed for the failure, does the post hoc fallacy come into play.

    Always investigate: but do so with an open mind.

    1. This is a great point. The key is to be careful not to fall foul of post hoc fallacy, but to still ensure you are investigating fully.

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