You're going to have a biased sample. If you want to determine the effectiveness rate, you'd need to ask for parents of children who've been treated with Lidcombe Program - regardless of their outcome. THEN see how many are still stuttering.Bias is not an issue, because I don't look at the global population.
At the moment your methodology is like saying you want to hear from people who've had a recurrence of cancer, after a course of chemotherapy, and concluding that chemotherapy is an ineffective treatment.
If the people die or relapse after chemotherapy, it says something after the chemotherapy. And the more stories I hear, the more concerning. I never hear that someone dies of a nose job or appendix removed. There IS information is that information.
If Lidcombe is 100% effective, no kid will be stuttering any more. So if I find someone or a few, Lidcombe cannot be 100% effective.
Someone once commented, I think it was Peter Reitzes, that the adult stutterers are going to die out in Australia! We shouldn't have any stuttering teenagers any more.
Moreover, we can look at those cases, and look whether they are normal cases, i.e. parents were following instructions, and therapist was well-trained. If we have such cases who fail, we seriously need to ask whether it is effective.
I am just saying it's a cheap way to do proper research. Find and look at the failure. Therapist researchers mostly do research on what is successful.
Moreover, if I find 10 failed cases, I should have at least 90 successful cases. We then have a success rate of 90%, which is a bit above natural recovery.