A long time ago Voltaire said
Common sense is not so common.
and nothing much has changed. However, one guy’s common sense in the next guy’s idiocy. Common sense, for everyone, is very subjective really unless you put it into perspective by calculating the average of everybody’s idea of common sense, which can be tricky. Common sense is not about democracy.
C.E. Stowe said
Common sense is the knack of seeing things as they are, and doing things as they ought to be done.
Which adds a bit of precision to the definition: common sense is about facts. Facts will allow you to make the right decision – given of course, that your facts are straight and you know what the best decision is. But that’s a different story.
Testing is a lot about common sense; all the techniques are tools of the mind, unlike the practical tools used for programming. It does not matter how a system works, or if you test an application, a document, a process or a sandwich. The approach is the same: get all the facts, define the frame of reference, inspect and compare the expected or desired result with the actual result. Sounds easy. It only works if you get it all right, though. You need all the facts, you need to know the frame of reference exactly. You use logic to get there.
It gets harder if the edges of your frame are blurred. If your facts have multiple variables, possibly all interdependent. You might need to look into the future. This is where common sense comes in.
Don’t mix up common sense with experience. Common sense can be gained and improved by experience, but hardly taught. And remember, common sense is subjective; it means something else for you than it means to Bob. Boundary value analysis may come natural to you. You might believe as the highly experienced programmer Bob is, he would know everything about it as well. Hey, you’re assuming here! Isn’t the tester’s mantra Never To Assume?
As much as you trust your “tester instincts”, as precise as your estimates might be, as trustworthy your risk analysis turns out – common sense will not save you, not more than pure logic will. Because common sense is not so common, because logic and people do not always mix. Measure, step back, use your common sense and measure again. It’s about both, facts and sense. If you neglect any, you will either end up deadlocked or eviscerated.
I realize my posts have been somewhat esoteric and elusive lately. I will try to focus on more practical things after the summer break, promised. I will also explain what I meant with my other post about sacrificing quality which came across wrong, apparently.