The short answer to the question, "What education and/or professional associations must a contractor have?" is none. All your contractor is required to possess is a current contractor's license and valid insurance. Anything else, though, is an indication that a contractor has gone above-and-beyond in an effort to expand his knowledge and build his business.
The Home Improvement Web provides a comprehensive list of contractor associations. These groups work to educate contractors in a particular niche and to provide them with ongoing information and networking options. If a contractor you're considering working with is a member of a professional association, this site is an excellent source for learning more about what they do.
Because association membership is not mandatory in order for a contractor to work, ask to see pictures of his previous work. The old adage about a picture being worth a thousand words is especially true when it comes to contracting work.
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