Evidence-appraisal glossary
Conditional Power
The probability that a trial will finish with a statistically significant result, calculated partway through using the data seen so far plus an assumption about the remaining effect. It guides interim decisions about futility.
Also called: conditional power.
At an interim look, conditional power asks: given what we have observed and a supposition about the true effect, how likely is the study to succeed if it runs to completion? A low value, often below an agreed threshold, signals that continuing is unlikely to pay off and supports stopping for futility. The answer depends heavily on which assumed effect is plugged in, whether the original target or a value estimated from current data, so the same trial can look more or less promising by that choice. It is a monitoring aid, not a verdict on the treatment.
Read the full Reading the Evidence blog.
This is a plain-language methodology definition for reading research. It is general education, not medical advice.