Just a note; the vast majority of bad test results are from operator/user error, no matter who the operator/user is, unless they are well experienced
This is why years ago, Habib from Salifert, sent out free test kits. All came with their own test samples. Some testers knew what the test samples where suppose to be and some where not told. The reasoning was to find what the operator error was. All testers should have come out with same resuts and some did and some did not. On known samples they just palyed with things like light, background colors and more exact sample volume until they go it right.