After putting an edit through we need to start making some checks. So we need some checking techniques and we need to make sure we're always auditing the work we're doing. If you put 10 edits through and you only then notice an error chances are you're never going to find where that error is come from. So after each edit we need to a sense check does the output make sense other numbers going up as expected or down as expected? We haven't got any weird items like 0 suddenly popped up. We then want to do a structure check make sure the formula structure is consistent and as expected. For instance if I'm writing a formula into column G. You might expect it to link to other column G cells and lastly a stress check does the model react predictably to changes if you put a very large increase in revenue through do you get a very large increase in net income? Other checks include checking that the balance sheet balances, check every sheet after each edit to make sure nothing untoward has happened, and check for error values. These are probably the most obvious things that will go wrong things like #div/0 #value #ref and #num.