If the treatment is applied in higher doses or more frequently (at least up to a point), the improvement should be greater or occur sooner. Because there may be other causes of improvement beside the treatment in question, the improvement must be shown in multiple patients in multiple settings and in circumstances where as many other possible causal factors can be ruled out.
Assessments of the efficacy of specific treatments typically start with some evidence of biological plausibility. Basic laboratory studies or other kinds of knowledge that do not involve direct tests of a treatment in live human patients may suggest that a treatment or class of treatments should work. No matter how compelling the arguments for plausibility, though, plausibility per se is not evidence for treatment efficacy.
The randomized controlled trial (RCT) is the most reliable methodology for assessing the efficacy of treatments in medicine. In such a trial a defined group of study patients is assigned to either receive the treatment or not, or to receive different doses of the treatment, through a formal process of randomization. A coin flip is the simplest example of a random process. In a study with two “arms” (e.g., treatment or no treatment), each eligible patient would receive whatever a coin flip indicated—heads for treatment and tails for no treatment. In a large number of patients, any clinical or demographic factors such as age, height, weight, illness history, other illnesses, or any other unknown factor that might affect the results of the treatment would be equivalent in the two groups. These will all be eliminated, then, as plausible competing explanations for any observed difference in outcome between the two groups.
Randomized trials typically include other features that increase the strength of the conclusions about cause-and-effect relationships between the treatment and the outcome of interest. Some patients may be excluded from the study because they have conditions that make it impossible to evaluate outcomes or gather data (e.g., extremely elderly patients may be excluded from a study of a cancer treatment because too many of them would die of other conditions before the end of a five-year follow-up period). The study of the efficacy of a drug may include lab tests that measure the level of the drug in the bloodstream. This is done to ensure that the patients assigned to the treatment group actually received the drug while the patients randomized to “no treatment” did not take it on their own. A study may include near-term clinical measures of benefit (e.g., reduction in blood pressure or cholesterol level) as well as long-term objective measures of benefit (e.g., remissions of tumors, mortality) or long-term subjective measures of benefit (e.g., self-reported pain or functional status levels).
Even though an RCT provides strong evidence for or against the efficacy of a given treatment being tested, no one study is ever so perfect that the results cannot be challenged. A study may show an absence of an