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Developing and implementing specifications for purchases of raw materials is a preventive control, as it should limit the purchase of defective raw materials.
Determination of spoilage after production is complete will not minimize defects caused by poor quality raw materials. It is at best a detective control.
Timely follow-up on unfavorable usage variances is at best a detective control, which may identify poor quality materials that may be causing unfavorable usage variances. It will not minimize defects caused by the poor quality raw materials.
While proper handling of work-in-process inventory is important, this will not ensure that raw materials are not of poor quality, and it will not minimize defects caused by poor quality raw materials.