One of the main problems of data mining is imperfection of input data. Such data may be uncertain, vague, and incomplete. In our data set, describing preterm birth, many attribute values were missing, that is, the input data set was incomplete. The main approach to solving the missing attribute value problem was based on a closest fit: a missing attribute value in a case was replaced by the existing attribute value in the best candidate, a case that fits as closely as possible (resembles the most) the case with the missing attribute value. We experimented with three methods based on the idea of the closest fit: looking for the best candidate among the set of all cases, among the cases that belong to the same concept (cases within the same class as the case with missing attribute values), and a special method, where the set of all attributes was restricted to a single attribute with the missing attribute value. In the last method, the missing attribute value was replaced by the most common value within the concept for symbolic attributes, and by the average value of all attribute values of the same concept for numerical attributes.