Typically there are three main categories of performance data presented to you by Performance Management products: measured, calculated and modeled. Be very wary of any products that try to blur the lines between the three!

Measured data offers values that have been measured directly from the operating system. Third party performance products obtain this data from either the operating system meters or from utilities supported by the manufacturer that themselves take the data from the meters. As both sources are the same, each is as accurate an approach as the other. The latter benefits from quicker and easier porting between operating system releases for the supplier. Also, the utilities approach means that the third party is not introducing additional risk by loading software onto your machine that accesses the operating system kernel.

Calculated data are values derived from measured data. This is done to 'fill in' gaps where the operating system does not produce the metrics required. Clearly the relevant screens or associated Help text should state what values are measured and what are calculated. Often calculations rely on assumptions made by the third party supplier. The quality of these assumptions directly affects the quality of the data. Any supplier that won't 'come clean' about where and what its assumptions are must naturally be treated with some suspicion! Even worse, if they can't come clean!

modeled data relates specifically to capacity planning products that use the 'analytical modeling' approach to performance prediction. This technique is invaluable in predicting relative change in response time for a given change in the environment e.g. hardware upgrade or workload increase. Put alongside measured data without explanation however, and it is thoroughly misleading. As managers often want response time data, you could end up embarrassed if you mistakenly give them figures purporting to be measured - especially if the predicted values don't match user experience!

The rules are simple. Know where the data you are using comes from, and don't trust a supplier that tries to hide the facts!