Formal code inspections are about twice as efficient as any known form of testing in finding deep and obscure programming bugs and are the only known method to top 80 percent in defect-removal efficiency. ...
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“Users make great testers. Don’t worry—if it’s wrong, they’ll tell us eventually.”
The best time to find problems in code is as soon as the code is written. If you let code sit and rot for a while, it won’t smell any prettier.