But these trials cannot rule out the possibility of constant biases— selection biases that affect each condition under study. Constant bias does not threaten inferences about relative effectiveness of one treatment over another across randomized conditions. But it does threaten inferences about the absolute size of any treatment effect, relative to zero, making cost-effectiveness or benefit-cost estimation hazardous. Moreover, constant biases make it difficult to accurately forecast the likely effects of expanding treatment coverage to include those not currently receiving services. To some extent, these inferential threats can be addressed through sophisticated statistical methods (discussed in a later section). But ultimately, the most persuasive strategy for addressing these concerns is the use of randomized trials with a no-treatment control group. According to Higgins (1999:517):
However, even in [recent] controlled trials, the absence of “placebo” or no-treatment control groups precludes precise estimates of what proportion of pre- to post-treatment changes are attributable to treatment. The cocaine-dependence treatment field would be well served by careful consideration of what additional experimental or quasi-experimental control conditions might be ethically and practically possible in future efficacy and effectiveness studies to help strengthen the validity of causal inferences and permit more precise estimates of the contribution of treatment to any changes observed.
The almost complete lack of no-treatment control groups in drug treatment research is striking. While there are numerous studies of placebo versus a new medication plus minimal counseling, studies of patients randomized to nothing at all are lacking. The drug treatment community has not ignored this issue (e.g., Anglin and Hser, 1990) but has generally responded with three plausible objections: clients are unlikely to agree to possible assignment to a no-treatment control group, no-treatment control groups are unethical, and there are alternative methods for achieving the same inferences.
Will clients agree to randomization to a no-treatment control group? The committee agrees that this is an important practical concern, but in the absence of such trials for drug treatment, it is not possible to estimate the magnitude of the problem. Presumably, this is a greater concern for studies of clients voluntarily seeking treatment than for studies of legally coerced clients (discussed below).
Are no-treatment control groups unethical? In brief, many clinicians argue that it is unethical to withhold treatment from those in need of it. Of course, one might counter that this begs the question of whether in fact drug treatment is beneficial. There is a competing ethical concern—the missed opportunities involved in failing to discover a more effective treatment because of undue faith in current standards of best practice.