A long paper trail of software engineering studies has shown that many internal code metrics (such as methods per class, depth of inheritance tree, coupling among classes etc.) are correlated with external attributes, the most important of which is bugs. What the authors of this paper show is that when they introduce a second variable, namely, the total size of the program, into the statistical analysis and control for it, the correlation between all these code metrics and bugs disappears.
via Vivek Haldar – Size is the best predictor of code quality. Essentially length of code tied with code metrics becomes a reasonable predictor of bugs.