Clinical trial is the primary research vehicle the healthcare sector. The most commonly performed clinical trials evaluate new drugs, medical devices, biologics, or other interventions on patients in strict scientifically controlled settings. It should be noted that a clinical trial determines efficacy not effectiveness. In medical jargon, effectiveness is how well a treatment works in practice and efficacy is how well it works in a clinical trial.

Research also includes research in non-medical departments such as Psychology, Business, Anthropology and Sociology; however in these fields the research deliberately looks for aggregate responses, not individual ones. In these areas investigators may not be evaluating drugs, treatments or devices but may be evaluating different approaches, methods, or experiences to produce new knowledge about an area.

Clinical Study Types:

Clinical study types divide into those relating to efficacy (which measures how well treatment works in clinical trials or laboratory studies) and those relating to effectiveness (effectiveness relates to how well a treatment works in practice)

(In order from strongest to weakest empirical evidence inherent to the design when properly executed.)

  1. Experimental Studies: The hallmark of the experimental study is that the allocation or assignment of individuals is under control of investigator and thus can be randomised. The key is that the investigator controls the assignment of the exposure or of the treatment but otherwise symmetry of potential unknown confounders is maintained through randomisation. Properly executed experimental studies provide the strongest empirical evidence for the specific interventions within the conditions of the study. The randomisation also provides a better foundation for statistical procedures than do observational studies.
    1. Randomised Controlled Clinical Trial (RCT): A prospective, analytical, experimental study using primary data generated in the clinical environment. Individuals similar at the beginning are randomly allocated to two or more treatment groups and the outcomes the groups are compared after sufficient follow-up time. Properly executed, the RCT is the strongest statistical evidence of the probable clinical efficacy of preventive and therapeutic procedures in the clinical setting.
    2. Randomised Cross-Over Clinical Trial: A prospective, analytical, experimental study using primary data generated in the clinical environment. Individuals with a chronic condition are randomly allocated to one of two treatment groups, and, after a sufficient treatment period and often a washout period, are switched to the other treatment for the same period. This design is susceptible to bias if carry over effects from the first treatment occur. An important variant is the “N of One” clinical trial in which alternative treatments for a chronically affected individual are administered in a random sequence and the individual is observed in a double blind fashion to determine which treatment is the best.
    3. Randomised Controlled Laboratory Study: A prospective, analytical, experimental study using primary data generated in the laboratory environment. Laboratory studies are very powerful tools for doing basic research because all extraneous factors other than those of interest can be controlled or accounted for (e.g., age, gender, genetics, nutrition, environment, co-morbidity, strain of infectious agent). However, this control of other factors is also the weakness of this type of study. Animals in the clinical environment have a wide range of all these controlled factors as well as others that are unknown. If any interactions occur between these factors and the outcome of interest, which is usually the case, the laboratory results are not directly applicable to the clinical setting unless the impact of these interactions are also investigated.
  2. Observational Studies: The allocation or assignment of factors is not under control of investigator. In an observational study, the combinations are self-selected or are “experiments of nature”. For those questions where it would be unethical to assign factors, investigators are limited to observational studies. Observational studies are considered in some cases to provide weaker empirical evidence than do experimental studies because of the potential for large confounding biases to be present when there is an unknown association between a factor and an outcome. The symmetry of unknown confounders cannot be maintained. Whilst observational studies provide limited evidence of efficacy, dependent on size, they may provide strong empirical evidence.  In relation to complementary therapies, evidence of effectiveness is of greater significance than evidence of efficacy. The greatest value of small scale types of studies (e.g., case series, ecologic, case-control, cohort) is that they provide preliminary evidence that can be used as the basis for hypotheses in stronger experimental studies, such as randomized controlled trials. On the large scale they can expose weakness in the results of experimental studies and provide better evidence of real-world clinical effectiveness.
    1. Cohort (Incidence, Longitudinal Study) Study: A prospective, analytical, observational study, based on data, usually primary, from a follow-up period of a group in which some have had, have or will have the exposure of interest, to determine the association between that exposure and an outcome. Cohort studies are susceptible to bias by differential loss to follow-up, the lack of control over risk assignment and thus confounder symmetry, and the potential for zero time bias when the cohort is assembled. Because of their prospective nature, cohort studies are stronger than case-control studies when well executed but they also are more expensive. Because of their observational nature, cohort studies do not provide empirical evidence of efficacy that is as strong as that provided by properly executed randomised controlled clinical trials of a similar size.
    2. Case-Control Study: A retrospective, analytical, observational study often based on secondary data in which the proportion of cases with a potential risk factor are compared to the proportion of controls (individuals without the disease) with the same risk factor. The common association measure for a case-control study is the odds ratio. These studies are commonly used for initial, inexpensive evaluation of risk factors and are particularly useful for rare conditions or for risk factors with long induction periods. Unfortunately, due to the potential for many forms of bias in this study type, case control studies provide relatively weak empirical evidence even when properly executed.
    3. Ecologic (Aggregate) Study: An observational analytical study based on aggregated secondary data. Aggregate data on risk factors and disease prevalence from different population groups is compared to identify associations. Because all data are aggregate at the group level, relationships at the individual level cannot be empirically determined but are rather inferred from the group level. Thus, because of the likelihood of an ecologic fallacy, this type of study provides weak empirical evidence.
    4. Cross-Sectional (Prevalence Study) Study: A descriptive study of the relationship between diseases and other factors at one point in time (usually) in a defined population. Cross sectional studies lack any information on timing of exposure and outcome relationships and include only prevalent cases.
    5. Case Series: A descriptive, observational study of a series of cases, typically describing the manifestations, clinical course, and prognosis of a condition. A case series provides weak empirical evidence because of the lack of comparability unless the findings are dramatically different from expectations. Case series are best used as a source of hypotheses for investigation by stronger study designs, leading some to suggest that the case series should be regarded as clinicians talking to researchers. Unfortunately, the case series is the most common study type in the clinical literature.
    6. Case Report: Anecdotal evidence. A description of a single case, typically describing the manifestations, clinical course, and prognosis of that case. Due to the wide range of natural biologic variability in these aspects, a single case report provides little empirical evidence to the clinician. They do describe how others diagnosed and treated the condition and what the clinical outcome was.
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