  Background: Test-driven development (TDD) is a technique that repeats short
coding cycles interleaved with testing. The developer first writes a unit test
for the desired functionality, followed by the necessary production code, and
refactors the code. Many empirical studies neglect unique process
characteristics related to TDD iterative nature. Aim: We formulate four process
characteristic: sequencing, granularity, uniformity, and refactoring effort. We
investigate how these characteristics impact quality and productivity in TDD
and related variations. Method: We analyzed 82 data points collected from 39
professionals, each capturing the process used while performing a specific
development task. We built regression models to assess the impact of process
characteristics on quality and productivity. Quality was measured by functional
correctness. Result: Quality and productivity improvements were primarily
positively associated with the granularity and uniformity. Sequencing, the
order in which test and production code are written, had no important
influence. Refactoring effort was negatively associated with both outcomes. We
explain the unexpected negative correlation with quality by possible prevalence
of mixed refactoring. Conclusion: The claimed benefits of TDD may not be due to
its distinctive test-first dynamic, but rather due to the fact that TDD-like
processes encourage fine-grained, steady steps that improve focus and flow.
