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Glossary: “ … a lexicon of the technical, obscure, or foreign words of a work or field.” 1
Each issue of Evidence-Based Mental Health will include a glossary introducing technical and obscure words used in different fields. The first glossary covered terms used in diagnosis. This issue will cover terms used in treatment. We will use the article by Kendall et al (May issue p 43),2 whose results are summarised in the table⇓ below. In this study, 60 children with an anxiety disorder were randomised to receive cognitive behaviour therapy (CBT) and 34 were randomised to a waiting list control. After 8 weeks of treatment, 53% (32 of 60) of the children receiving CBT were free of their primary anxiety disorder; this is referred to as the experimental event rate (EER). The control event rate (CER) refers to the proportion of children in the control group also free of anxiety disorder after 8 weeks of being on a waiting list.
Terms used in therapeutics
WHEN THE EXPERIMENTAL TREATMENT INCREASES THE PROBABILITY OF A GOOD EVENT:
RBI (relative benefit increase): the increase in the rates of good events, comparing experimental and control patients in a trial, also calculated as |EER − CER|/CER. Using the figures from the study by Kendall et al, RBI is (32/60 − 2/34)/(2/34) = 807%. In other words, there is roughly an 800% increase in rates of being free of anxiety disorder in the CBT group compared with the waiting list control.
ABI (absolute benefit increase): the absolute arithmetic difference in event rates of a positive outcome, |EER − CER|. In this example, ABI is 53% − 6% or 47%, or put another way, of 100 children treated, 47 more children are free of anxiety disorder if treated with CBT compared with a waiting list control.
NNT (number needed to treat): calculated as 1/ABI, and denotes the number of patients who must receive the experimental treatment to create one additional improved outcome in comparison with the control treatment. The NNT in the study by Kendall et al is 1/.47 = 2.13. We usually round up the NNT to the next highest integer, which is 3 in this case. In other words, 3 children with anxiety disorder need to be treated withCBT in order to achieve one more child free of anxiety disorder compared with a waiting list control. The lower the NNT, the more effective the intervention.
WHEN THE EXPERIMENTAL TREATMENT REDUCES THE RISK OF A BAD EVENT (SUCH AS PREVENTING RELAPSE), THE SAME CALCULATIONS CAN BE USED BUT WITH SLIGHTLY DIFFERENT TERMINOLOGY:
RRR (relative risk reduction): the proportional reduction in rates of bad events between experimental (EER) and control (CER) participants in a trial, calculated as |EER − CER|/CER.
ARR (absolute risk reduction): the absolute arithmetic difference in event rates, |EER − CER|.
NNT: the number of patients who need to be treated to prevent one additional bad outcome, calculated as 1/ARR.
There are also circumstances when an intervention can lead to a series of unfavourable outcomes; long term neuroleptic use may cause tardive dyskinesia; treatment of conduct disorder may lead to stigmatisation. In these circumstances, there may be risks associated with the intervention instead of benefits and the intervention may do more harm than good.
WHEN THE EXPERIMENTAL TREATMENT INCREASES THE PROBABILITY OF A BAD EVENT:
RRI (relative risk increase): the increase in rates of bad events, comparing the experimental patients to control patients in a trial, and calculated as |EER − CER|/CER.
ARI (absolute risk increase): the absolute difference in rates of bad events, when the experimental treatment harms more patients than the control treatment, and calculated as |EER − CER|.
NNH (number needed to harm): the number of patients who, if they received the experimental treatment, would lead to 1 additional person being harmed compared with patients who receive the control treatment, and calculated as 1/ARI.
Confidence interval (CI): the CI quantifies the uncertainty in measurement; usually reported as 95% CI, which is the range of values within which we can be 95% sure that the true value for the whole population lies.
Terms used in diagnosis (see glossary in Evidence-Based Mental Health 1998 Feb for more detail)
THE FOLLOWING TERMS ARE USED IN COMPARING A NEW TEST AGAINST A DIAGNOSTIC (GOLD) STANDARD:
Prevalence: the proportion of people in the sample who have the disorder.
Sensitivity: the proportion of people who have the disorder (according to the diagnostic [gold] standard) who are detected by the test.
Specificity: the proportion of people who do not have the disorder (according to the diagnostic [gold] standard) who are determined by the test to not have the disorder.
Positive predictive value: the proportion of people who score positive on the test who actually have the disorder.
Negative predictive value: the proportion of people who score negative on the test who actually do not have the disorder.
Likelihood ratio for a positive test result: the likelihood that a positive test comes from a person with the disorder rather than one without the disorder = sensitivity / (1 – specificity).
Likelihood ratio for a negative test result: the likelihood that a negative test comes from a person with the disorder rather than one without the disorder = (1 – sensitivity) / specificity.
THE FOLLOWING TERMS ARE USED WHEN 2 OR MORE RATERS ARE COMPARED WITH EACH OTHER, AND NONE IS CONSIDERED TO BE A GOLD STANDARD:
Crude agreement: the proportion of cases for which all raters agree.
Kappa (κ): an index of agreement “corrected” for the fact that raters will agree with each other a certain proportion of the time just by chance.
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