Related Experiment Video
Updated: Aug 11, 2026
04:53
A Clinical Trial Assessing the Safety, Efficacy, and Delivery of Olive-Oil-Based Three-Chamber Bags for Parenteral Nutrition
Published on: September 20, 2019
Review of randomised trials using the post-randomised consent (Zelen's) design
Joy Adamson1, Sarah Cockayne, Suezann Puffer
1York Trials Unit, Department of Health Sciences, University of York, York YO10 5DD, UK.
Contemporary Clinical Trials
|February 4, 2006
Summary
Zelen's method randomizes participants before consent to reduce bias, a common reason for its use. Despite ethical concerns, recent trials show manageable crossover rates, suggesting its careful application can be effective.
Area of Science:
- Clinical Trials Methodology
- Biostatistics
- Medical Ethics
Background:
- Zelen described a pre-consent randomization method in 1979 to improve clinical trial recruitment.
- This method faces ethical criticism regarding consent and scientific critique over high crossover rates.
- This review examines recent trials employing Zelen's method, their justifications, crossover rates, and overall quality.
Purpose of the Study:
- To review recent trials utilizing Zelen's method.
- To identify authors' stated reasons for employing this randomization technique.
- To assess crossover rates and the quality of trials using Zelen's method.
Main Methods:
- A literature review was conducted, searching for trials citing Zelen's papers since 1990.
- Personal knowledge was used to include additional relevant trials.
- The review focused on identifying justifications, crossover data, and trial quality.
Main Results:
- 58 relevant trials were identified.
- The primary justification for using Zelen's method was to prevent bias, such as the Hawthorne effect.
- While 41 trials reported crossover (median 8.9%), it was generally within acceptable limits; few trials explicitly aimed to enhance recruitment.
Conclusions:
- Limiting bias, including the Hawthorne effect and 'resentful demoralization', was the main reason authors cited for using Zelen's method.
- Careful implementation of Zelen's method can mitigate bias in clinical trials.
- Contrary to previous reviews, this study found crossover rates to be manageable in most trials.
Related Concept Videos
01:13Randomized Experiments
The randomization process involves assigning study participants randomly to experimental or control groups based on their probability of being equally assigned. Randomization is meant to eliminate selection bias and balance known and unknown confounding factors so that the control group is similar to the treatment group as much as possible. A computer program and a random number generator can be used to assign participants to groups in a way that minimizes bias.
Simple randomization
Simple...
Simple randomization
Simple...
01:11Blinding
Blinding is a commonly used method of not telling participants which treatment a subject is receiving. Blinding is a critical part of a randomized control trial or RCT. It reduces the bias that affects the results. In an RCT, blinding is used in the form of a placebo. A placebo effect occurs when untreated subjects falsely believe they have received the treatment and report improved symptoms. A placebo or a dummy treatment is administered to subjects to negate the bias caused by such an effect.
02:07Blind Procedures
Ideally, the people who observe and record the children’s behavior are unaware of who was assigned to the experimental or control group, in order to control for experimenter bias. Experimenter bias refers to the possibility that a researcher’s expectations might skew the results of the study. Remember, conducting an experiment requires a lot of planning, and the people involved in the research project have a vested interest in supporting their hypotheses. If the observers knew which child was...
01:16Experimental Designs
An experimental design is a systematic process that allows researchers to evaluate the relationship between dependent and independent variables. There are three widely used types of experimental design - pre-experimental design, true experimental design, and quasi-experimental design. In pre-experimental design, the researcher compares the data before and after some interventions or treatments. The true-experimental design has more than one purposefully created group, a commonly measured...
02:01Group Design
The most basic experimental design involves two groups: the experimental group and the control group. The two groups are designed to be the same except for one difference— experimental manipulation. The experimental group gets the experimental manipulation—that is, the treatment or variable being tested—and the control group does not. Since experimental manipulation is the only difference between the experimental and control groups, we can be sure that any differences between the two are due to...
01:23Types of Biopharmaceutical Studies: Controlled and Non-Controlled Approaches
Biopharmaceutical studies constitute a vital field aiming to enhance drug delivery methods and refine therapeutic approaches, drawing upon diverse interdisciplinary knowledge. In research methodologies, the choice between controlled and non-controlled studies significantly influences the study's reliability and accuracy.
Non-controlled studies, commonly employed for initial exploration, lack a control group, rendering them susceptible to biases and external influences. In contrast, controlled...
Non-controlled studies, commonly employed for initial exploration, lack a control group, rendering them susceptible to biases and external influences. In contrast, controlled...