![]() Once the primary outcome measure has been set, the remaining efficacy and adverse effect outcomes comprise the secondary outcome measures. Alternatively, to save himself the bother, he could instead designate improvement in working memory as the primary outcome measure. He sets a composite cognitive index, formed from mathematically combining the results of these neuropsychological assessments, as the primary outcome measure. He identifies a large battery of neuropsychological tests, including tests of attention, concentration, working memory, logical memory, visuospatial memory, ideational fluency, perceptuomotor speed, and problem solving. He wishes to find out whether risperidone is associated with better cognitive outcomes. In another example, an investigator plans to conduct an RCT that compares risperidone with haloperidol in patients with schizophrenia. However, this would not be a good idea because a much larger sample size would be necessary to identify statistically significant differences for categorical outcomes (eg, response rate) as compared with continuous outcomes (eg, reduction in MADRS scores). In this RCT, the investigator may choose, instead, to designate response rate (defined as 50% attenuation of MADRS scores) as the primary outcome measure. In other words, he sets improvement on the MADRS as the primary outcome in the RCT the result on this single outcome is the primary determinant of whether the study is considered a “success” or a “failure.” All of the remaining assessments are hierarchically less important and comprise the secondary outcomes. Out of this long list, the investigator decides that improvement on the MADRS is the most important if the antidepressant drug attenuates MADRS ratings significantly more than does placebo, he will conclude that the drug is effective in treating MDD. He also plans to record vital physiologic parameters such as the heart rate and blood pressure and obtain electrocardiograms and routine laboratory tests.Īll of these are outcome measures, or dependent variables. He decides that he will administer the Montgomery-Asberg Depression Rating Scale (MADRS), the Hamilton Depression Rating Scale (HDRS), the Clinical Global Impression (CGI) scales for Severity (CGI-S) and Improvement (CGI-I), and instruments that measure quality of life, sexual functioning, and medication adverse effects. As an example, an investigator plans to compare a new antidepressant drug with placebo in an 8-week RCT in patients with major depressive disorder (MDD). The primary outcome measure is the variable that an investigator considers to be the most important among the many dependent variables that are to be examined in the study. This article explains what a primary outcome measure is, why it is necessary to specify the primary outcome a priori, how one may interpret the primary and secondary outcomes reported in a research article, and limitations of the concept of primary and secondary outcomes. ![]() Research articles that describe randomized controlled trials (RCTs) usually but not always specify a primary outcome measure. © Copyright 2015 Physicians Postgraduate Press, Inc. This article discusses the setting of the primary outcome measure, the need for it, the increased risk of false-positive and false-negative errors in secondary outcome results, how to regard articles that do not state the primary outcome, how to interpret results when secondary outcomes are statistically significant but not the primary outcome, and limitations of the concept of a primary outcome measure in clinical trial research. There are 2 reasons for this: it reduces the risk of false-positive errors resulting from the statistical testing of many outcomes, and it reduces the risk of a false-negative error by providing the basis for the estimation of the sample size necessary for an adequately powered study. The primary outcome needs to be defined at the time the study is designed. The primary outcome measure is the outcome that an investigator considers to be the most important among the many outcomes that are to be examined in the study.
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