Any study of TCO must include all relevant costs in order to be reliable. Go over the numbers for the studies that examine cost issues to make sure they look realistic. For either return on investment (ROI) or TCO, make sure the study authors covered all the bases. For a brief refresher on TCO and what should be included, see the first article in this series, Getting the Real Facts: How to Read an Analyst's Report.

One of the Microsoft-sponsored studies actually omitted a good number of client access licenses (CALs), which are typically a large portion of the licensing cost. This is a serious oversight, resulting in a misleadingly low cost for the Windows deployment.

Whether intentional or due to oversight, omissions of related costs in ROI and TCO studies are inexcusable and will yield warped findings.

Lesson: omissions can make the data unrealistic in practice.

Next: Is the Methodology Clear? »