When one say’s Quality, it usually is in context of the end user.
To start with lets take Juran’s definition of Quality . “Fitness for use.” by which two possible areas of measurement would be
- degree of Fitness for use.
- degree of Deviation from fitness for use. (complement of above)
While bug can be viewed as a deviation from Fitness for use, As per Myers “testing is the process of executing a program with the intent of finding errors. “
Testing detects bugs which are fixed by developers, As long as these bugs are fixed by the developers there is a definite increase in degree of Fitness for use or definite reduction in degree of deviation from fitness for use.
With the above one can safely state that, As long as developers fix bugs found by testers there is a definite improvement ( change for better) in quality.
Another perspective to the above will be, the major hindrance to quality in software is injection of defects, Defect injection can be minimized by defect prevention techniques & defect detection techniques.
Testing is a defect detection technique, it does not prevent defect injection but surely prevents slippage of a lot of defects to production, thereby improving quality experienced by the customer .