A second experiment in the application of quality engineering to sweet-potato cultivation was carried out. The first experiment took two years: an L18 orthogonal array experiment was carried out in the first year and then a confirmatory experiment was carried out in the following year. In the second experiment, the confirmatory experiment was carried out under conditions prepared in advance so that the orthogonal array experiment and the confirmatory experiment could be carried out simultaneously, which enabled the whole experiment to be finished within one year. The sweet potatoes were evaluated by analysis of variance;calculations were performed to determine interaction effects among months of cultivation, the size of the harvested potatoes, and relevant control factors and noise factors. As a result, it was found that months of cultivation and potato size are closely related. Relationships between months of cultivation and potato size were derived for each control factor level, enabling cultivation conditions to be selected for harvesting sweet potatoes of any desired size.
