The article analyzes three possible outcomes of an intervention study testing a new drug: improvement, deterioration, or no effect. Any result may be true, but there is always a risk of chance influence, lack of study sensitivity, or bias. Prejudices can strengthen or weaken a response, creating an apparent positive or negative effect, when in reality no change will occur. It can also be a combination of prejudices. Biases affect all types of clinical trials, not just interventional ones.