Why a written instruction is not enough on its own
A site instruction is essential and it is not sufficient. It cannot convey which door needs lifting to lock, how long the floor takes to dry in the rear corridor, that the consulting rooms are released at a particular time, or which cupboard belongs to the practice rather than to the cleaning. Those things are learned in an hour of working the building and are practically impossible to write down in a form anyone will read.
They also compound. A cleaner who does not know three small things about a building loses time on each, arrives at the end of the shift behind schedule, and drops something. The client sees a poor clean and concludes the replacement is not competent. In fact the replacement was competent and unprepared, which is a different problem with a different and much cheaper solution. The distinction between incompetent and unprepared matters, because only one of them is expensive to fix.
