A lighting company pursues to study the percentages of defective glass shells being manufactured. Ideally the percentages of defectives are dependent on temperature, humidity as well as the level of artisan expertise. Complete historical data are obtainable 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 defective also depend on production supervisors. But data on supervisors in charge are available for only 242 of the 365 days. How must this study be conducted?