First, let's define the key term. By consultant, we mean someone who offers point expertise in a specialized field for a limited duration, usually to address a critical need. You call a consultant when you're happy to pay for someone's deep knowledge, to avoid costly mistakes or quickly recover from a mistake you've already made.
Think everyone knows what a consultant is? We encountered some confusion in our research--particularly from vendors who offered to take over staffing functions for, say, PC management. Sorry, that's outsourcing.
As columnist David Willis, an analyst with the Meta Group, puts it: "Consulting is what you want when your need is strategic. Outsourcing is what you want when your need is not strategic." Consultants are usually hired to improve or fix, but never to maintain a process or function.