This paper examines randomized controlled trials (RCTs) as the gold standard in causal research, explaining how random assignment controls for subject variables and why double-blind designs further reduce bias. It outlines the core limitations of RCTs, including ethical constraints on random assignment, the impracticality of true control groups in certain medical studies, and the need for larger sample sizes. The paper also addresses the distinction between internal and external validity, discussing how sampling methods and real-world applicability affect a study's generalizability. Finally, it considers when stratification of known confounds is preferable to relying on random assignment alone.
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Randomized controlled trials (RCTs) are considered the "gold standard" in research that attempts to identify a causal relationship between an independent variable and a dependent measure (Kraemer, Wilson, Fairburn, & Agras, 2002). The reason for this is that the process of random assignment to treatment groups — or to the groups that receive the independent variable — statistically controls for all individual differences among participants that could affect the outcome, except for the variable of interest (Kraemer et al., 2002). The control condition in an RCT allows for comparisons between the different levels of the independent variable and no intervention at all. This is particularly important in medical studies where a condition such as depression or illness may change over time without any type of intervention or treatment.
RCTs are not a remedy for poorly designed research methodologies, and they do not eliminate experimenter bias, participant expectations, or measurement errors (Jackson, 2012). In order to reduce experimenter bias and participant expectations, double-blind RCTs — in which neither the researchers nor the participants know which condition of the experiment they have been assigned to — are themselves considered the "gold standard" (Kraemer et al., 2002).
There are several drawbacks to true experimental designs that use randomized assignment (e.g., see Jackson, 2012; Kraemer et al., 2002; Simon, 2001). First, sample sizes typically need to be larger than in many types of correlational research. Second, certain subject variables cannot be randomly assigned — gender or a congenital condition, for example. When examining specific influences on why people develop a particular disorder or attitude, researchers cannot randomly assign subjects to conditions in which they would develop problems such as drug abuse; doing so would be unethical, as human subjects cannot be deliberately placed in conditions that cause harm or disorder.
In cases where diseases such as HIV are being investigated, it is also not ethical to use a pure control group in which individuals with HIV receive no intervention. In these situations, new medications are compared to existing medications, and there is no true control group. RCTs are similarly not appropriate for research studies that are not examining specific cause-and-effect relationships.
Finally, the use of randomized assignment and blind experimental conditions allows researchers to infer cause-and-effect relationships only within the particular study conditions in which the research takes place. RCTs do not allow researchers to generalize their findings beyond the sample of participants in the specific study.
This brings up the question of internal and external validity in research. The internal validity of a research study describes how well its conditions control for extraneous variables that could influence the findings. RCTs typically have good internal validity. External validity refers to the ability of a particular research study to generalize beyond its own conditions. External validity depends on how researchers recruited their participants (the sampling method used) and how well the study's conditions emulate the real-world conditions to which the findings are meant to be applied (Jackson, 2012).
For instance, Inal and Kelleci (2012) investigated the use of distraction to decrease procedural pain in children between the ages of six and twelve years. The findings of that study would not generalize to adults over 21, as these two cohorts differ substantially in developmental and psychological characteristics.
"When stratification supplements random assignment for accuracy"
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