It doesn't matter if you are selecting an external evaluator because you are participating in a program that requires one or you are looking to employ the services of an evaluator because you have too much on your plate and simply don't have time to handle program evaluation alone. In either case, you are faced with the task of making a selection that will have a significant impact on the future of your programs and you'll want to make a wise choice. We have narrowed the list of things you'll need to consider to a few ABCs.
Availability. How available is the external evaluator to take on your project? Of course, you want someone with experience, but you also want someone who has time to handle your project in a professional manner and work with you throughout the year. Just because an evaluator accepts your contract does not mean that she really has time for it. Ask how many other projects the evaluator will be working with and how she knows that she will have sufficient time to give your project the time it deserves.
Background. What experience has the evaluator had with projects like yours? How much specific evaluation experience has she had? How much formal education in evaluation and data analysis has the evaluator received? What experience has the evaluator had in education? You need someone who has some experience working as an evaluator, but you also need someone who has had some good general background in your field because all of the data that she will be working with exists within the very complex context of the educational system. Someone who has not worked within the context may have trouble meeting your needs. I have seen two mistakes made in this area: 1) hiring a well-known educator or scholar who has no experience in evaluation and 2) hiring someone with evaluation experience who has never worked with non-profits or schools (or an organization like yours). Try to avoid those situations.
Cost. How much will the evaluator's services cost? Evaluator fees vary tremendously. A high fee does not necessarily mean better service; however, keep in mind that most professional evaluators are in private business. That means that if they were not in demand, they would not be able to command a high fee. Someone who is offering services at a bargain basement rate is likely not in demand and there is probably a reason for that. So you need to be wary of both high and low fees. A reputable evaluator will be able to explain her fees and will be more than happy to provide you with an itemized breakdown of fees on your request. Also, remember that you can negotiate. If your budget does not allow a fee as high as the one that the evaluator quoted, you can suggest a modification of work (for example, maybe you will do the surveys yourself and you really only want two on-site visits per year, instead of four) that could also result in a modification of the fee. Not all evaluators are amenable to this, but it never hurts to ask.