Behind the Research with Jamie Holloway: What’s an Arm?

Go behind the research with Jamie Holloway, and learn more about the clinical trial process.

Behind the Research is an ongoing series to peek behind the research bench and learn more about the words and phrases that scientists use. We hope that through a better understanding of the science of disease and how clinical research works, you will be a more empowered and confident participant in your care.

In this installment, we’re going to take a look at some of the words that people use to describe clinical research studies and the way participants are grouped. They’re not words that most of us use in everyday conversation, but they’re pretty specific and descriptive, so there aren’t good substitutes.

Arm: I think we all know what an arm is, but in the world of clinical trials, “arm” rarely refers to the body part that attaches to your hand. In a clinical trial, patients are divided into groups, and each group is called an arm.

Control Arm: This is a group of patients who are either given a placebo or standard of care treatment in a clinical trial. They match the patients in the investigational arm as closely as possible when it comes to disease state, gender, ethnicity, and prior treatment. Because these two groups are so well matched, it makes it easier to determine which effects (both good and bad) are caused by the investigational drug.

Investigational Arm: Also sometimes called the treatment group, this is the group of patients in the clinical trial who receive the investigational drug.

Investigational Drug: This is the drug that is being studied in a clinical trial. Sometimes it is called the experimental drug. Since the goal of a clinical trial is to determine the safety and efficacy of a new drug, the researchers are investigating its value, thus, it is considered an investigational drug.

Longitudinal Study: A longitudinal study follows a group of participants over an extended period of time. Because these are long-term studies, they are frequently used to assess the impact of risk factors on the development of disease or the impact of a treatment over time. While a longitudinal study will often span years or even decades, there is no length requirement — the key factor is that the outcome can be assessed as it changes over time.

Observational Study: An observational study is one where researchers observe outcomes in different groups without doing anything to affect the outcome. It is not a clinical trial: participants are observed and health outcomes are monitored, but they are not given any intervention or treatment as part of the study.

Randomized Control Trial (RCT): An RCT is considered the gold standard of clinical trials. These are usually very large trials that are designed to ensure that the researchers can see even quite small impacts of the investigational drug. In an RCT, patients are randomly assigned to be in either the control or investigational arm, and then their response is followed to determine how effective the investigational drug is and what side effects patients on the drug might expect.

This started as an idea for a single post, but it quickly became evident that the best plan would be to aliquot the idea into multiple posts to form a series. If there’s a word, phrase, or concept that you’d like me to tackle in a future installation of our Behind the Research series, please leave a comment and I’ll add it into my queue!


Jamie Holloway is a both a scientist and a survivor, earning her PhD in tumor biology from Georgetown University — where she spent long hours researching breast cancer — a few years before her own breast cancer diagnosis. Now living with no evidence of disease after treatment for early stage triple negative breast cancer, she bridges the gap between scientists and researchers as a Clinical Research Advocate for Science 37, and as the Patient Advocate for the Metastatic Breast Cancer Project at the Broad Institute. She works with researchers as part of the Georgetown Breast Cancer Advocates and writes about her personal experience with cancer on her blog Run Lipstick Chemo, and as a contributor to the Cure Magazine community. A wife, mother, runner, and lipstick addict, Jamie shares her story from the perspective of both a patient and a scientist.

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