LSEbA: least squares regression and estimation by analogy in a semi-parametric model for software cost estimation. The importance of Software Cost Estimation at the early stages of the development life cycle is clearly portrayed by the utilization of several models and methods, appeared so far in the literature. The researchers’ interest has been focused on two well known techniques, namely the parametric Regression Analysis and the non-parametric Estimation by Analogy. Despite the several comparison studies, there seems to be a discrepancy in choosing the best prediction technique between them. In this paper, we introduce a semi-parametric technique, called LSEbA that achieves to combine the aforementioned methods retaining the advantages of both approaches. Furthermore, the proposed method is consistent with the mixed nature of Software Cost Estimation data and takes advantage of the whole pure information of the dataset even if there is a large amount of missing values. The paper analytically illustrates the process of building such a model and presents the experimentation on three representative datasets verifying the benefits of the proposed model in terms of accuracy, bias and spread. Comparisons of LSEbA with linear regression, estimation by analogy and a combination of them, based on the average of their outcomes are made through accuracy metrics, statistical tests and a graphical tool, the Regression Error Characteristic curves