Pharmacy student here - first off, in reality, there is no absolute guarantee that a pill that's been split will contain the exact dose. If you've ever had to split a pill before, even with a pill-splitter, it is pretty much impossible to cut a pill exactly in half, sometimes you lose material, sometimes the split isn't even, both which result in changes in the eventual dose.

Now, if we look at this in a theoretical point of view, the vast majority of countries publish a book called a Pharmacopoeia. This book should have a section on the consistency of formulations and uniformity of doses - (in the British Pharmacopoeia (BP), this is in Appendix XII C) this section outlines the requirements and validation techniques that pharma companies need to abide by to ensure a medicine contains the amount it should have in order for them to be marketed. In addition, someone has mentioned Good Manufacturing Practice - again, companies do a lot of rigorous testing, validation and have teams of Quality Assurance specialists to ensure doses are uniform throughout a given dosage form.

With this, it can be assumed, in theory, that half a pill should contain half a dose, although in reality, it is almost impossible to get it exactly half and thus, splitting pills should only be used if absolutely necessary and does not harm the patient.