A cognitive bias is the human tendency to make systematic errors in judgment, knowledge, and reasoning. Such biases can result from information-processing shortcuts called heuristics. They include errors in statistical judgment, social attribution, and memory. Cognitive biases are a common outcome of human thought, and often drastically skew the reliability of anecdotal and legal evidence. It is a phenomenon studied in cognitive science and social psychology.

An example of a cognitive bias is the presentation of statistics. You would feel very differently if you were told by a doctor that you had a particular disease, but 90% of people survived compared to if he told you 1 in 10 people died.

A prosecutor might frame things in a specific way to try and obtain a particular result. Or someone presenting evidence might unwittingly do the same.