Editing Sampling bias (section)

===Symptom-based sampling===
The study of medical conditions begins with anecdotal reports. By their nature, such reports only include those referred for diagnosis and treatment. A child who can't function in school is more likely to be diagnosed with [[dyslexia]] than a child who struggles but passes. A child examined for one condition is more likely to be tested for and diagnosed with other conditions, skewing [[comorbidity]] statistics. As certain diagnoses become associated with behavior problems or [[intellectual disability]], parents try to prevent their children from being stigmatized with those diagnoses, introducing further bias. Studies carefully selected from whole populations are showing that many conditions are much more common and usually much milder than formerly believed.