Reliability

The headless professor discusses the concept of reliability and some of the more common types of reliability measures.

He presents a 2 x 2 contingency table to demonstrate reliability as having more agreements than disagreements. What do measures of reliability determine? Why did the headless professor present the 2 x 2 table to explain reliability?

How is reliability defined in that instance? Why is a type of inter-rater reliability important for clinicians?