Testing subject lines can help email marketers increase conversion rates. Email consultant Jeanne Jennings explained how she helped her client increase conversion rates by 19 percent by doing so. In a piece for ClickZ, she explained the process, " we scheduled the test over multiple sends, to try to guarantee we'd get statistically significant results (it's a small group; there are only about 40,000 subscribers total). Once again we pitted the control subject line ('Subscriber Newsletter') against subject lines that included a key theme in the first 25 characters, your subject line 'prime real estate.'"