In tons of cases you are in fact "certain." You don't need an experiment to know that pointer chasing is going to be slower than array iteration, or that in so

In tons of cases you are in fact "certain." You don't need an experiment to know that pointer chasing is going to be slower than array iteration, or that in some circumstance pulling a function out of a loop might be better but won't be substantially worse than the compiler's output. Generally speaking, the cases where you are unsure, it's sensitive to platform details or unstable compiler optimizations, or you don't know what sort of user input you'll really get. Or in the case of the blog post, if you don't have a good model of the execution environment in your head (because Python).