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Levels of Evidence for Human Studies of Cancer Complementary and Alternative Medicine (PDQ®)

Strength of Study Design

In the classification system, a numeric scale from 1 to 4 is used to indicate the statistical strength of the study design, with 1 assigned to studies having the strongest design and 4 assigned to studies having the weakest design. Further subdivision of some design categories yields finer measures of strength. The various types of study design are described below in descending order of strength:

  1. Randomized controlled clinical trials: Studies in which participants are assigned by chance to separate groups for the comparison of different treatments. It is the patient's choice to be in a randomized trial, but neither the researcher(s) nor the patient can choose the group in which he or she will be placed. Using chance to assign people helps to ensure that the groups will be similar and that the treatments they receive can be compared objectively. At the time of a trial, there is uncertainty about which of the treatments is best. These trials can be "double-blinded" or "nonblinded." Double-blinded trials have a stronger study design.
    1. Double-blinded: Neither the patients nor the researcher(s) know which patients are receiving the therapy under study or the comparison (i.e., control) treatment.
    2. Nonblinded: The researcher(s) and the patients know what treatment is being given.
  2. Nonrandomized controlled clinical trials: Studies in which participants are assigned to a treatment group based on criteria that may be known to the researcher(s), such as the patient's birth date, chart number, or day of clinic appointment. With this type of study design, there is less confidence that the group receiving the treatment under study and the control group are comparable.
  3. Case series: Studies that describe results from a group or series of patients who all received the treatment that is being investigated. These studies have a weak design, due, in part, to the absence of a control group. Different types of case series, in descending order of strength, are as follows:
    1. Population-based, consecutive case series: The study population is well-defined and is either the entire population of interest or a representative random sample of the larger population from which it is drawn. The study subjects receive treatment in the order in which they are identified by the researcher(s).
    2. Consecutive case series: Studies describing a series of patients who were not limited to a specific population and who received treatment in same order in which they were identified by the researcher(s).
    3. Nonconsecutive case series: Studies describing a series of patients who were not limited to a specific population and who do not represent a consecutive series of patients identified and treated by the researcher(s).
  4. Best Case Series: From a larger series of patients, only the cases that appear to have benefited from the treatment under study are reported. These studies have the weakest design.
  • Updated: August 26, 2010