By Dean Rieck, direct mail copywriter
One of the primary strengths of direct mail is that it gives you an easy and accurate way to measure results and improve your marketing efficiency. However, to get actionable testing results, you must approach the testing process with logic and discipline. 
Here are a few tips for getting the full value out of your direct mail tests.
• Make knowledge your #1 goal. Yes, you want to boost response and increase net income now. However, building a body of knowledge about what people want and what they respond to is ultimately more important. Testing is about gaining knowledge. Profits come from the application of that knowledge. Don’t just look for “what” works, try to figure out “why” things work. This allows you to apply your knowledge intelligently. 
• Don’t rely too heavily on what others are doing. You must always run your own tests, because every company, market, product, and offer is in some way unique. Learn from others, but consider it a starting point, anecdotal information at best. Most people don’t test as carefully as they should, so just because you frequently see a particular direct mail piece doesn’t mean you can count on it being a winner.
• Establish a solid control, then test. Your first priority is to create a control so you can test lists, media, offers, prices, product configurations, and other big issues. Then, you move on to control testing to boost results. Usually, this means you should stick to the basics at first, then try more creative ideas after you have some solid response data and a record of success. 
• Run statistically valid tests. A change in results and a statistically valid change in results are two different things. If you don’t know the mathematics of testing, hire someone who does. When you get a statistically valid difference, re-test to confirm your results. 

• Have a goal for every test. What do you want to learn? Write down the questions you would like answered and run a test for each. Ideally, every test should answer one specific question. 
• Test one element at a time. Testing more than one element will make it impossible for you to know what makes a difference in results. If you’re testing price, change only the price in your mailer. Don’t change anything else. If you constantly test elements to refine a control, you are taking what I call the “Evolutionary Approach,” slowly and methodically growing a stronger and stronger control over time. 

• Test an all-new approach. If you don’t test a single element — whether for lack of patience or the desire for a big breakthrough — form an alternate hypothesis and test a completely different creative treatment. The more different the better. Reworking the same approach may change results, but won’t give you the breakthrough you’re looking for. This is what I call the “Revolutionary Approach,” going for the big win all at once.
• Track results meticulously. Keep detailed reports on the number of pieces mailed, the number of responses you get, response source, the conversion percentage, the income those responses generate, the average order, the percentage response, your income per thousand, your cost per order or cost per response, your net dollar return, bad debt, returns, and every other fact that you need to calculate specifically how your direct mail performs. Over time, this will be a gold mine of information.
• Use your tests to determine strategy. Mathematical results are the best way to end debates on issues such as creative approaches, formats, and offers. Never assume. Always test. No one knows better what will work than your own prospects and customers.
• Keep using your control until you beat it. Never, ever shelve a control just because you’re tired of it. If it’s making money, keep using it. Your control isn’t there to entertain you or win you awards. It’s there to generate income. Until you have the figures in black and white that conclusively demonstrate that you have a new control, keep that old one going.
• Keep testing. Testing is a long-term commitment. It’s not something you do once and forget. It’s not something you do only when you have a little extra in your budget. Testing should be — must be — part of your routine. You want to follow what I call the Testing Loop: 1. Run a test. 2. Analyze the results. 3. Act based on the results. 4. Repeat.
—Source: Dean Rieck is a leading direct mail copywriter. For more copywriting and selling tips, sign up for Dean’s FREE direct response newsletter and get a free report, 99 Easy Ways to Boost Your Direct Mail Response.