Reliability & Validity Defined
Reliability and validity are two desirable qualities of any measurement procedure or instrument. There is no such thing as perfect reliability or validity. Even measures that we think of as accurate will always have some source of error.
Reliability is the extent to which an "experiment, test, or any measuring procedure yields the same results on repeated trials."2 The tendency towards consistency in repeated measurements is its reliability. So, even though Ms. Jones blood pressure yielded three different readings when taken by your nurse, the medical student and you, they are close. One is not sky high and the others low. There is reliability between the three readings.
Validity is the extent to which the construct measures what it says it is measuring. The use of a blood pressure cuff is considered to be valid because it is measuring blood pressure, not something else. Using an opthalmoscope to measure blood pressure would not be a valid method.
How do I determine if my measurements are reliable and valid?
In order to determine if your measurements are reliable and valid, you must look for sources of error. There are two types of errors that may affect your measurement, random and nonrandom.
Random error consists of chance factors that affect the measurement. The more random error, the less reliable the instrument.
The type of reliability assessed in this example is retest reliability. This is called the coefficient of stability. It is expressed as a correlation coefficient (r) which will range from 0 to 1. The closer to 1, the more reliable the measurement.
Non-random error is systematic. If the blood pressure cuff always reads high, then it affects all of the measurements. Non-random error affects the validity of the instrument.
The type of validity assessed in this example is that of construct validity. The blood pressure cuff measures the construct as it is defined in the literature. Thus, it measures what it claims to measure.
A measurement can be highly reliable and yet not valid. For example, an alarm clock that is set for 7AM but rings every morning at 6:30AM is reliable, but not valid