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    ABSTRACT: Lack of concealment of allocation in randomized clinical trials can invite selection bias, which is the preferential enrollment of specific patients into one treatment group over another. For example, patients more likely to respond may be enrolled only when the next treatment to be assigned is known to be the active treatment, and patients less likely to respond may be enrolled only when the next treatment to be assigned is known to be the control. Despite the fact that selection bias can compromise both the internal and external validity of trials, little methodology has been developed for its detection. An investigator may test the success of the randomization by comparing baseline characteristics across treatment groups, but such test is limited by the potential inability of the measured baseline variables to predict response. A new method for detecting selections bias, based on response data only, is developed for the case in which a small block size, and either unmasking of treatment codes or an open-label design, have compromised the concealment of allocation. This new method complements baseline comparisons, and is sensitive to detect selection bias even in situations in which baseline comparisons are not.
    Controlled Clinical Trials 09/1999; 20(4):319-27. DOI:10.1016/S0197-2456(99)00014-8
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    ABSTRACT: Active upper-GI bleeding (spurting or oozing) or a visible vessel at endoscopy are high-risk lesions that predict recurrence of bleeding. The aim of this study is to determine whether nasogastric aspirate predicts the presence of high-risk lesions. The Canadian Registry of patients with Upper Gastrointestinal Bleeding undergoing Endoscopy was used to identify patients with upper-GI bleeding who underwent nasogastric aspiration and subsequent endoscopy. An association between nasogastric aspirate findings (bloody, "coffee ground," clear/bile) and high-risk lesions was sought. Of 1869 patients in the registry, 520 had documented nasogastric aspiration before endoscopy. Those who underwent aspiration did not differ from those who did not. A bloody nasogastric aspirate was significantly associated with high-risk lesions (odds ratio 4.82: 95% CI[2.3, 10.1] vs. clear/bile; and odds ratio 2.8: 95% CI[1.8, 4.3] vs. coffee ground). A bloody nasogastric aspirate had the highest specificity for high-risk lesions (75.8%: 95% CI[70.0, 80.0]) with a negative predictive value of 77.9%: 95% CI[73.2, 82.0], and raised the probability of having a high-risk lesions from 0.29 to 0.45. A clear nasogastric aspirate reduced the likelihood to 0.15. Nasogastric aspirate yielded the most useful information in hemodynamically stable patients without hematemesis. Nasogastric aspirate is useful in predicting high-risk lesions. Whether it can be used to determine which patients would benefit from earlier endoscopy deserves further study.
    Gastrointestinal Endoscopy 03/2004; 59(2):172-8. DOI:10.1016/S0016-5107(03)02543-4 · 5.37 Impact Factor
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    BMJ (online) 05/2006; 332(7547):969-71. DOI:10.1136/bmj.332.7547.969 · 17.45 Impact Factor