PubMed · 5936919
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J H Seaber, W F Case. 1966. Oculomotor imbalance. A classification for computer programming.. https://pubmed.ncbi.nlm.nih.gov/5936919/
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This article discusses Rush-Presbyterian-St. Luke's Medical Center's approach to assessing and preventing errors in care and promoting patient safety. The word error is applied to all kinds of events, including adverse occurrences, negligence, and malpractice. Thus confusion exists among those analyzing the causes of adverse events. A patient safety committee standardized the definition of medical error and developed a taxonomy for error as a prelude to efforts at error reduction. It identified three levels or layers that can represent a train of events culminating in an undesired outcome: error, treatment failure, and adverse event. This discussion is offered in the interest of clarifying some of the issues.
It sometimes happens (for instance in case control studies) that a classifier is trained on a data set that does not reflect the true a priori probabilities of the target classes on real-world data. This may have a negative effect on the classification accuracy obtained on the real-world data set, especially when the classifier's decisions are based on the a posteriori probabilities of class membership. Indeed, in this case, the trained classifier provides estimates of the a posteriori probabilities that are not valid for this real-world data set (they rely on the a priori probabilities of the training set). Applying the classifier as is (without correcting its outputs with respect to these new conditions) on this new data set may thus be suboptimal. In this note, we present a simple iterative procedure for adjusting the outputs of the trained classifier with respect to these new a priori probabilities without having to refit the model, even when these probabilities are not known in advance. As a by-product, estimates of the new a priori probabilities are also obtained. This iterative algorithm is a straightforward instance of the expectation-maximization (EM) algorithm and is shown to maximize the likelihood of the new data. Thereafter, we discuss a statistical test that can be applied to decide if the a priori class probabilities have changed from the training set to the real-world data. The procedure is illustrated on different classification problems involving a multilayer neural network, and comparisons with a standard procedure for a priori probability estimation are provided. Our original method, based on the EM algorithm, is shown to be superior to the standard one for a priori probability estimation. Experimental results also indicate that the classifier with adjusted outputs always performs better than the original one in terms of classification accuracy, when the a priori probability conditions differ from the training set to the real-world data. The gain in classification accuracy can be significant.
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