Understanding Clinical Trial Results

How to read trial outcomes in plain language: endpoints, statistical significance, and what results mean for you.

By Victor Lafforgue, Founder of TrialsAlert. See our editorial policy for how we source and review content.

What are clinical trial results?

Clinical trial results are the findings that emerge from testing a new treatment in human participants. They tell us whether the treatment works, how well it works, and what side effects it causes. Understanding these results helps patients and caregivers evaluate whether a treatment is promising, how it compares to existing options, and what it might mean for their own care. Results are typically published in medical journals and registered on ClinicalTrials.gov. Not all results indicate success. Negative results that show a treatment does not work are equally important for advancing medical knowledge and preventing patients from receiving ineffective treatments.

Primary and secondary endpoints

Every clinical trial defines specific measurements called endpoints to evaluate whether the treatment works. The primary endpoint is the main outcome the trial was designed to measure, for example, overall survival, progression-free survival, or reduction in symptoms. The trial is considered successful or unsuccessful based primarily on this measurement. Secondary endpoints provide additional evidence, such as quality of life improvements, time to response, or duration of response. A treatment might meet its primary endpoint but not all secondary endpoints, or vice versa. Both types of results are important for understanding the full picture of how a treatment performs.

Statistical significance and p-values

A p-value is a statistical measure indicating the probability that the observed result happened by chance. A p-value below 0.05 is generally considered statistically significant, meaning there is less than a 5% probability the result is random. However, statistical significance does not always mean clinical significance. A treatment might produce a statistically significant improvement that is too small to meaningfully affect a patient's life. When evaluating trial results, look at both the p-value and the actual magnitude of the benefit.

Hazard ratios and number needed to treat

A hazard ratio (HR) compares the rate of an event between the treatment group and control group. An HR of 0.75 means the treatment group had 25% fewer events (such as disease progression or death) compared to the control group. Lower is better. The number needed to treat (NNT) tells you how many patients need to receive the treatment for one additional patient to benefit. An NNT of 5 means that for every 5 patients treated, 1 will benefit who would not have with the standard treatment. Lower NNT indicates a more effective treatment. An NNT of 5 is generally considered good, while an NNT above 20 suggests limited benefit for individual patients.

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