Answer to Question 1
Reliability refers to whether you will get similar measurements from a participant if you make repeated observations. Validity refers to whether the measurements you make are really useful in helping you understand the concepts you are studying.
Measurements can be reliable without being valid. Just because you get the same results when you test an individual repeatedly, it does not necessarily mean that you are making the right measurements. On the other hand, if you have valid results they must be reliable because if you are measuring what you want to measure, you should be able to measure in consistently, that is, reliably.
Answer to Question 2
When physicians make diagnoses, they call upon their own experience to interpret the behaviors of patients. If a person behaviors in a way different than the physician expects, it could lead to a diagnosis based on a mismatch between behavior and expectation, not on underlying physical or psychological problems. In the case of the patient who suffered from carbon monoxide poisoning (described in Controversy Box 1), the initial physician made inappropriate assumptions about what the patient knew, so when the patient didn't conform to expectations, the doctor erred in his diagnosis.
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