What is the probability of making a type 1 error commonly set at?

The probability of making a Type I error — which occurs when a researcher incorrectly rejects a true null hypothesis — is commonly set at 0.05. This means that there is a 5% risk of concluding that a difference or effect exists when there is none. This threshold is widely accepted in many fields, including psychology, because it represents a balance between being too lenient and too strict in hypothesis testing.

Having the probability set at 0.05 helps researchers to control the chance of making this error while still allowing for some level of discovery in their research findings. In contexts where the consequences of a Type I error are particularly critical, such as in medical research, the threshold may be set even lower, such as at 0.01. However, 0.05 remains the most common standard in psychological research.

Understanding this probability is essential for interpreting statistical results and for the replicability and reliability of findings in clinical psychology and other disciplines.