Which is it? Is the conventional wisdom correct — that one out of every 10 or 12 public radio listeners is a station member? Or is it the more encouraging one-in-three, as found by the Audience 98 research project?The seemingly conflicting estimates flew past each other at last month’s Public Radio Development/Marketing Conference in Washington, D.C., without much elucidation. Now comes an attempt at elucidation. The leading proponents of the 1:12 ratio, Oregon-based fundraising consultants Lewis-Kennedy Associates, reported at the conference that an average of 8.3 percent of stations’ weekly cume listeners can be found as donors in the membership files.