As long as it's not a hugely noisy in the way it changes within each exchange/source of inventory, you should be able to account for it. Don't forget plenty of

As long as it's not a hugely noisy in the way it changes within each exchange/source of inventory, you should be able to account for it. Don't forget plenty of other "legitimate" changes happen every day to affect the value of placements too - usage patterns, page layouts, tastes and fashions etc. They can be just as nefarious but all need to be accounted for - fraud is just another factor.