Let us say that two physicians each examine the knees of 100 patients in an arthritis clinic. After each examination they record whether they believe each patient’s knee to be swollen or not.

• Physician A finds that 30 of the patients have swollen knees.
• Physician B finds 40 of the patients have swollen knees.

The physicians agreed that 25 of the patients had swollen knees, and that 55 patients did not (leaving 15 patients where the doctors disagreed).