over a period of 6 months i would not expect the readings to change in a ideal world. a domestic property has a maximum of 10 years between periodic tests. if it is rented it should be tested when there is a change of renter.there are factors which can cause test result changes. ie damage,wear tear,corrosion,excessive electrical loading ageing etc.it is the installation as a whole which is tested.so it is hard to say as things can go wrong. but on the other hand i have seen people just write in results without even testing. the test equipment should be calibrated.if the test results vary on a