If what you want to know is "what characteristics do LW participants and ex-participants have" then you want to survey LW participants and ex-participants, and responses from other people will not help to answer that question. And if their survey answers don't clearly distinguish the other people from the participants and ex-participants, they will make the answers less useful. I forget how clearly the questions ought to make it possible to distinguish, but given that people make mistakes and don't fill everything in I suspect that in practice they distinguish much less than perfectly.