Many people refer to actions or decisions as “common sense.” However, manners of thinking and behaving typically develop based on the subjective reality of individuals, and these may not be so sensible. They often arise from cognitive bias. 

According to the Agency for Healthcare Research and Quality, about 75% of diagnostic mistakes include an element of cognitive bias. 

Anchoring bias 

A patient may mention the most serious symptom first, leading a doctor to correctly diagnose a health condition. However, patients do not always know which symptoms are most important or which ones may not be relevant to the illness at all. 

Anchoring, or focalism, may lead a physician to give too much weight to the first symptom mentioned and interpret all the other symptoms in light of that one rather than assessing their individual relevance. Anchoring could even cause doctors to ignore new information after forming the initial impression. 

Confirmation bias 

When a physician already has strong personal beliefs or theories regarding care, he or she is likely to interpret new information in a way that confirms those beliefs and theories. This could lead doctors to ignore what may seem obvious to health care providers who approach the symptoms open-mindedly. 

Confirmation bias often serves to support or create anchoring bias errors, as the physician may ask leading questions or ask the wrong questions when examining the patient. 

Availability bias 

The New York Times reports that doctors may also make bad decisions or diagnostic errors because of a recent adverse event. For example, a patient may suffer an uncommon and serious side effect from a medication. The doctor may hesitate to prescribe that same medication again, even though it may be exactly what the next patient needs. 

One study revealed that obstetricians were more likely to use a different delivery mode if something went wrong during the previous birth. So, if a patient suffered an adverse event during a C-section, a doctor may recommend vaginal delivery for the next patient regardless of safety.