Validity of a test or measurement
relates to the accuracy of a method to measure what it is intended to
measure. Because measurement error is related to the reliability of a
test, one could argue that a test must be reliable in order to be valid.
Yet, an invalid test can be reliable. For instance, the results of a
testing methodology may be reproducible, but not accurately measuring the
variable of interest.
To illustrate the link between
reliability and validity of a measurement, consider the analogy of shooting an
arrow at a target. Shooting the arrow at the bulls-eye of the target
would represent performing the test. The bulls-eye represents the
characteristic of the variable being measured.
A tight shot pattern suggests that the
test produces reproducible results (high reliability).
A shot pattern that is spread
"wide" suggests that the test produces results that are not
consistently reproducible (low reliability).
A shot that hits the bulls-eye suggests
that the test produces results that reflect the true characteristic of the
variable being measured (high validity).
A shot that misses the bulls-eye
indicates that the test produces results that do not reflect the true
characteristic of the variable being measured (low validity).
A tight shot pattern that consistently
hits the bulls-eye indicates that the results of a test are reliable and are
measuring the true characteristic of the variable being measure (high
reliability and high validity).
A test with acceptable accuracy
demonstrates an acceptable degree of reliability and validity. One can
now understand that "acceptable" should be defined, meaning what is
an acceptable degree of reliability and validity. The definition of
"acceptable" is important since testing methods rarely display
perfect reliability and validity. Acceptable reliability and validity of
a test is sometimes debatable, but should be based on evidence-informed
decisions.
Validity is based on the categories of
evidence that can be used to support the validity of a test. Below are
descriptions of various types of validity with links to example studies.
Face Validity
Indicates that a testing
methodology appears to measure what it is supposed to measure.
Content Validity
Indicates that the various components
of the testing methodology include the content of the variable being measured.
Criterion-related Validity
Indicates that the results of a test
can be used as a substitute for the findings of reference-standard test.
Concurrent Validity
Indicates that the results of a test are
valid when compared to reference-standard test; established when both tests are
performed at relatively the same time (concurrently); a form of
criterion-related validity.
Predictive Validity
Indicates that the results of a test
are valid since the test results can predict a future outcome; another form of
criterion-related validity.
Construct Validity
Indicates that the results of a test
reflect an abstract construct.
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