Validity & Reliability

Validity
These two terms, reliability and validity, are often used interchangeably when they are not related to statistics. When critical readers of statistics use these terms, however, they refer to different properties of the statistical or experimental method.
Reliability is another term for consistency. If one person takes the same personality test several times and always receives the same results, the test is reliable.
A test is valid if it measures what it is supposed to measure. If the results of the personality test claimed that a very shy person was in fact outgoing, the test would be invalid.
Reliability and validity are independent of each other, a measurement maybe valid but not reliable, or reliable but not valid. Suppose your bathroom scale was reset to read 10 pounds lighter. The weight it reads will be reliable (the same every time you step on it) but will not be valid, since it is not reading your actual weight.
Reliability

Reliability is a very important part...