Mean Time Between Failure (MTBF) is often proposed as a key decision making criterion when comparing data center infrastructure systems. Misleading values are often provided by vendors, leaving the user incapable of making a meaningful comparison. When the variables and assumptions behind the numbers are unknown or are misinterpreted, bad decisions are inevitable. This paper explains how MTBF can be effectively used as one of several factors for specification and selection of systems, by making the assumptions explicit.