I am looking for possibly a study between the time difference of regular coding vs coding + unit tests (not strict TDD just yet). I know the whole "Saves you time in the long run" angle, but from a project planning perspective for the team that has never done it before, I need to be able to roughly estimate how much additional time to allocate.
Does such a study exist? Can anyone comment from experience?
I know that you're not interested in going for full TDD just yet, but I think the best performance and planning markers are going to be found in Test Driven Development case studies, such as those conducted by both Microsoft and IBM.
Case studies were conducted with three development teams at Microsoft and one at IBM that have adopted TDD. The results of the case studies indicate that the pre-release defect density of the four products decreased between 40% and 90% relative to similar projects that did not use the TDD practice. Subjectively, the teams experienced a 15–35% increase in initial development time after adopting TDD. Source.
That is part of a preface to a full comparison of normal development vs development using unit testing as a core principle. The upfront development planning addition that we use is 20%, which includes both unit and integration tests. In all honesty testing is vital to any successful piece of software, unit testing just removes some of the hard yards and manual effort. The ability to run a few hundred unit and integration tests upon finishing a bit of functionality and being able to test the system within a few seconds is invaluable.
There are a number of different 'magic' numbers that people add to their estimates to incorporate the additional time to write unit tests, but it really just comes down to a simple trade-off. Essentially your balancing the increase in development time vs the increase in bug fixing/error checking time, also taking into account the likeyhood of downtime and the system criticality (if it is a primary revenue or a non-essential system).
If you’re interested in doing a little more reading here is the complete Microsoft study. Its pretty short but gives some interesting findings. And if you’re really keen here is a unit testing slideshow that outlines the concepts and benefits in reasonable detail (previously this was a link to the source content of this presentation, but sadly that content is now gone).