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The base-rate fallacy occurs because learners often ignore the low prevalence of the disease (the base rate) and focus only on the high accuracy of the test. This leads to the incorrect assumption that a positive test result is highly likely to be a true positive, when in fact the large number of healthy people generates many more false positives than true positives.

Conditions: Prevalence is low ($1\%$).; Test accuracy (sensitivity/specificity) is high ($95\%$).; The observer focuses on test accuracy rather than population composition.

The base-rate fallacy occurs because learners often ignore the low prevalence of the disease (the base rate) and focus only on the high accuracy of the test. This leads to the incorrect assumption that a positive test result is highly likely to be a true positive, when in fact the large number of healthy people generates many more false positives than true positives.

Conditions: Prevalence is low ($1\%$).; Test accuracy (sensitivity/specificity) is high ($95\%$).; The observer focuses on test accuracy rather than population composition.