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
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
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.
1 | List 3 things that might have introduced random error into Ms. Jones blood pressure reading. Some possibilities are: person taking the reading time of day instrument might not be reliable |
2 | . What might you do to attempt to help establish reliability of Ms. Jones BP measurement? Take her blood pressure again. |
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.
3 | Are there any non-random sources of error possible in your assessment of Ms. Jones BP? Some examples are: situation induced - "white coat syndrome" wrong cuff for the patient cuff that always measures high or low |
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
Reliability and Validity
These two terms are sometimes used interchangeably in research and evaluations. However, they mean different things. Reliability and validity are concepts used to evaluate the quality of research. They indicate how well a method, technique or test measures something. Reliability is about the consistency of a measure, and validity is about the accuracy of a measure.
It’s important to consider reliability and validity when you are creating your research design, planning your methods, and writing up your results, especially in quantitative research.
What is Reliability?
Reliability [or precision] refers to consistency. That is, if you use an instrument or test several times, you should get the same results. If the data [or the instrument] are unreliable, then the data are considered unrelated to the phenomenon or the concept being measured. This, therefore, means that the results cannot be repeated. For example, a broken thermometer that gives a different measurement every time it is placed in the same environment under the same conditions is not reliable.
What is the Validity?
Validity simply means that a test or instrument is accurately measuring what it’s supposed to. In evaluations, we usually refer to two types of validity; internal and external.
Internal validity refers to the extent to which an instrument [or an evaluation] correctly answers the questions it claims to answer about what is being tested [or evaluated]. For example, a questionnaire [instrument] that asks persons to state the amount of their donations. Is the answer an indication of how charitable people are? Or is it their disposable income that is actually being measured by this instrument?
External validity refers to the extent to which the results of an evaluation can be generalised to other situations. That is, the extent to which the sample selection reflects the population. The value of external validity is the ability to generalise the results to a larger population.
A Final Word...
Tests or instruments that are valid are also reliable. E.g. a properly functioning thermometer is valid [and reliable] because it measures the correct temperature in a consistent manner every time. However, tests or instruments can be reliable but not always valid. E.g. , the broken thermometer that is a degree off would be reliable [giving you the same results each time] but not valid [because it was not recording the correct temperature].
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