As part of a project, I'm required to implement various protocols for randomisation of statistical trials. The most basic of these we call "simple": it's the equivalent of flipping a coin every item. This is generally enough for most purposes, although even with a perfectly weighted coin there is still the possibility of a variance between the number of heads and the number of tails. For a clinical trial, for statistical purposes it can be preferred that both heads and tails - in this case, experimental treatments, placebos and/or controls - end up flipping the same number of times, while
still requiring each individual result to be completely random.