This term describes knowledge or assumptions made based only on what one already knows before collecting data. It’s typically used to describe a starting hypothesis or the expectations researchers have at the start of developing a research question, before they have any other knowledge or evidence to go on.
Deeper Dive
In philosophy, a priori knowledge is that which a person has before they begin learning about the world — instead of a blank slate, a priori knowledge is the slate — so when used in science, it’s usually referring to a research question that hasn’t been explored yet or which the researchers have virtually no data to use in coming up with a hypothesis. A priori also refers to the status of a research question before a study begins in a more pragmatic sense, such as a priori registration of a clinical trial—registering it, most often at clinicaltrials.gov, before the trial begins.
A priori registration has become increasingly important as a way to ensure the credibility and transparency of the research process because it reduces the likelihood of p-hacking or changing a hypothesis or endpoint midway through the trial. The term a priori also refers more broadly to the state of knowledge or the data available before a trial versus after the trial (a posteriori), since some research papers explicitly investigate clinical relevance of a trial a priori vs. a posteriori, such as papers looking at findings’ generalizability.