Tester’s day

What the day is

Tester's Day is observed on September 9, marking a day in 1947 when operators of an early electromechanical computer at Harvard traced a fault to a moth caught in a relay. They removed it, taped it into the logbook, and noted that it was the first actual case of a bug being found.

Origin and background

The story is usually told as the origin of the word bug in computing, and that is the one part of it that is wrong. Engineers had been calling faults bugs for decades already. Thomas Edison used the term in correspondence in the 1870s to describe difficulties in his inventions, and it was ordinary shop language long before any computer existed.

Reading the logbook entry with that in mind makes it much better. The joke only works because everyone already said bug for a fault, so finding a literal insect was worth writing down and worth keeping. It is an engineers' pun, not a coinage, and the reason the page survives is that the people involved found it funny.

The day belongs to testers rather than to programmers, and testing has a logical limit worth stating. A test can demonstrate that a fault is present but can never demonstrate that no faults remain, since passing means only that nothing went wrong in the cases you happened to try. That asymmetry, put memorably by a Dutch computer scientist, is why testing is a matter of choosing cases well rather than running many of them.

Most of the useful craft follows from it. Faults cluster at boundaries, at zero, at empty inputs, at the maximum, and one past the maximum, and a small set of carefully chosen awkward cases finds more than a large set of ordinary ones.

Date handling remains among the most reliable sources of defects, as the Programmer's Day entry notes.

How to take part

• Thank a tester, who is usually the person delivering unwelcome news.
• Look up the logbook page, which is preserved and photographed.
• Test the boundaries: zero, empty, maximum, and one beyond it.
• Remember that passing tests prove nothing about what you did not try.