IT resources represent an important organizational asset and particularly useful for supporting decision making on business objectives. Software development activities have a significant role on the total of investments on IT resources. The Brazilian market for software and related services is growing every year and today it reaches almost 1% of the national GDP, offering opportunities for professionals and service providing organizations for software development. At the same time that turnover grows, results indicate that between 60% and 80% of the software projects exceed the amount of the estimate effort and/or duration, and it represents problems to organizations, clients and providers of this service. These problems are basically of two types and are associated to estimation errors of (i) effort and (ii) duration. This work has the objective of presenting an instrument for analyzing the impact factors for estimating errors of effort and duration in software projects. An exploratory and qualitative research was carried out by interviewing experts in software project management to validate the instrument. The paper details the instrument developing process taking into account the experts’ opinions about its significance and applicability. In its current stage of development, the instrument can already be considered an important tool for providing continuous improvement on software development projects.