A lighting company seeks to study the percentage of defective glass shells being manufactured. Theoretically, the percentage of defectives is dependent on temperature, humidity, and the level of artisan expertise. Complete historical data are available for the following variables on a daily basis for a year:
a. Temperature (high, normal, low).
b. Humidity (high, normal, low).
c. Artisan expertise level (expert, average, mediocre).
Some experts feel that defectives also depend on production supervisors. However, data on supervisors in charge are available for only 242 of the 365 days. How should this study be conducted?