Some individuals, notably young adult males, may not register with a doctor until they need to, which may be several months after they have moved, whilst others such as the elderly or pregnant women are likely to register immediately if not before they have made a move
Output data from the population census can be obtained at ward and enumeration district level
Some short distance moves that happen to cross an FHSA boundary will not be recorded because the individuals may not bother to register with a new doctor;