The purpose of this qualitative multiple case study is to research the current causes to cost estimation errors and revisit the research question of a well published quantitative study to explore whether responses would be consistent using qualitative methodology. Both overlaps and deviations are to be expected as a result of this comparison. The focus of this paper is on differences where new issues are likely to be found.The 8 resulting cross case cost estimation inhibitors validate 7 out of 16 issues found in the quantitative study. They are also related to the theory within and outside the area of software engineering resulting in five underlying software cost estimation inhibitors and providing a deeper understanding for the snapshot of the current causes of cost estimation errors reported by the practitioners.Lastly, the question of the practitioners' ability to report upon software cost estimation inhibitors is discussed. We argue that there might be causes of cost estimation errors that practitioners might be unwilling to reveal or might even be completely unaware of. Thus, they might unconsciously be providing researchers with invalid data.

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