Why randomized trials of NFP are rare

Here is an explanation of why randomized trials are rare, from a 2007 analytical study of a fairly large set of observational studies. (Hilgers et al.--the basis of Creighton--are "observational studies"):

"While we recognize that critics may argue that this study was not a randomized controlled trial, it should be recognized that very few FAB method studies are randomized controlled trials. The majority are observational studies. This study was a prospective cohort study which could be seen to be placed from an evidence-based perspective, between the retrospective case-control studies and the randomized clinical trials. The only randomized clinical trials on methods of NFP (Wade et al., 1981; Medina et al., 1980) are of limited use: they showed huge recruitment problems and retention as well as having a very strong selection bias (participants had to agree to expect quite high failure rates while attracted by free medical care at study entry), their results are therefore very questionable (Grimes et al. 2004). Randomized controlled trials are rarely used to investigate other family planning methods either because most couples have a preference for a certain method or do not wish to be randomized. In addition with most family planning methods, it is impossible to blind the couples from the allocated method unless comparing certain different hormonal contraceptives or intrauterine devices."
(P. Frank-Herrmann, et al., "The effectiveness of a fertility awareness based method to avoid pregnancy in relation to a couple's sexual behaviour during the fertile time: a prospective longitudinal study,"

Hum. Reprod. (2007) 22 (5): 1310-1319. doi: 10.1093/humrep/dem003)

This article argues that its own analytical method could be the best possible way of establishing evidence-based guidelines--but still not nearly as reliable as randomized trials.

Let's hope the good people at The Cochrane Collaboration find ways to analyze the evidence that do not require randomized trials!