\section{External Validity}

Our results make no use of local calibration data. That is,
our results are not biased by the historical record of different sites.
Hence, in this respect, our results score very highly in terms of external validity. 

However, in another respect, the external validity of our results is highly biased by the choice of underlying model (COCOMO, COQUALMO);
and the range of changes and projects we have explored:
\bi
\item {\em Biases from the model:}
While the above results hold over the entire space of calibration possibilities of COCOMO/COQUALMO, then may {\em not} hold
for different models.
One reason we are staying with
COCOMO/COQUALMO (at least for the time being) is that we have shown that STAR can be controlled these models without
requiring local calibration data. We find this to be a compelling reason to prefer these models. 
\item {\em Biased from the range of cases explored:}
Another threat to the external validity of our models is the range of changes explored in this study. This paper has only ranked
reuse against 
the changes listed in \fig{dchange}. 
Our changes may do better than and we will explore those in future work. 
\item {\em Biases from our selected case studies:}
Lastly, we have only explored the projects listed in \fig{cases}. We are currently working towards a more extensive study where
we explore more projects, including projects that do not come from NASA.
\ei
