If no large design data set is available to design the Multiple classifier system, one typically uses the same data set to design both the expert classifiers and the fusion rule. In that case, the experts form an optimistically biased training data for a fusion rule designer. We consider standard Fisher linear and Euclidean distance classifiers used as experts and the single layer perceptron as a fusion rule. Original bias correction terms of experts’ answers are derived for these two types of expert classifiers under assumptions of high-variate Gaussian distributions. In addition, noise injection as a more universal technique is presented. Experiments with specially designed artificial Gaussian and real-world medical data showed that the theoretical bias correction works well in the case of high-variate artificial data and the noise injection technique is more preferable in the real-world problems.