Ratio of positive to negative sample in data set for best classification
It is recommended to have around equal number of instances in each class. If not, then you should normalize the data. 4 or 5 times more positive class size could produce highly biased classifier. However, if your negative class is not well defined then there are ways to train your classes with only one class as mentioned in the paper, Learning from positive examples when the negative class is undetermined- microRNA gene identification.