PLEASE NOTE: We are currently in the process of updating this chapter and we appreciate your patience whilst this is being completed. Validity Validity is the extent to which an instrument, such as a survey or test, measures what it is intended to measure (also known as internal validity). This is important if the results of a study are to be meaningful and relevant to the wider population. There are four main types of validity:
Reliability Reliability is the overall consistency of a measure. A highly reliable measure produces similar results under similar conditions so, all things being equal, repeated testing should produce similar results. Reliability is also known as reproducibility or repeatability. There are different means for testing the reliability of an instrument:
Internal consistency can be measured using Cronbach’s alpha (a) – a statistic derived from pairwise correlations between items that should produce similar results. Ranging from minus infinity to one, a Cronbach’s alpha of one indicates perfect internal consistency, and a negative value suggests that there is greater within-subject variability than there is between subjects. Cronbach’s alpha values above 0.7 are generally deemed acceptable. Inter-rater reliability can be measured using the Cohen’s kappa (k) statistic. Kappa indicates how well two sets of (categorical) measurements compare. It is more robust than simple percentage agreement as it accounts for the possibility that a repeated measure agrees by chance. Kappa values range from -1 to 1, where values ≤0 indicate no agreement other than that which would be expected by chance, and 1 is perfect agreement. Values above 0.6 are generally deemed to represent moderate agreement. Limitations of Cohen’s kappa are that it can underestimate agreement for rare outcomes, and that it requires the two raters to be independent. Generalisability Generalisability is the extent to which the findings of a study can be applicable to other settings. It is also known as external validity. Generalisability requires internal validity as well as a judgement on whether the findings of a study are applicable to a particular group. In making such a judgement, you can consider factors such as the characteristics of the participants (including the demographic and clinical characteristics, as affected by the source population, response rate, inclusion criteria, etc.), the setting of the study, and the interventions or exposures studied. Threats to external validity, that may result in an incorrect generalisation, include restrictions within the original study (eligibility criteria), and pre-test/post-test effects (where cause-effect relationships within a study are only found when pre-tests or post-tests are also carried out). © Saran Shantikumar 2018 What property is established when the results of two separately administered tests that measure the same domain or concept are highly correlated?What property is established when the results of two separately administered tests that measure the same domain or concept are highly correlated? Convergent validity example: two or more instruments that theoretically measure the same construct and are administered to the same subjects.
What does concurrent validity indicate?Concurrent validity indicates the amount of agreement between two different assessments. Generally, one assessment is new while the other is well established and has already been proven to be valid.
What is the type of validity when an instrument produces results?This can take the form of concurrent validity (where the instrument results are correlated with those of an established, or gold standard, instrument), or predictive validity (where the instrument results are correlated with future outcomes, whether they be measured by the same instrument or a different one).
What is the term used to describe the consistency resulting from one person taking the same measure two or more times and getting roughly the same results?Reliability refers to the consistency of a measure. Psychologists consider three types of consistency: over time (test-retest reliability), across items (internal consistency), and across different researchers (inter-rater reliability).
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