14.11 A small random sample was used in an experiment to see how effective an informative newsletter was in persuading people to favor a flat income tax bill. Thirty persons were asked their opinion before receiving the letter and these same persons were then asked again after receiving the letter. Before the letter, 11 were in favor. Five were in favor both before and after receiving the newsletter, and six were opposed both times. Is there evidence that the letter is effective in persuading people to favor a flat tax?