The instability (θ) of the yield of sweet-potato has been calculated by the following formula, for each prefecture in Japan proper.
Where i and d is the rate of increased and decreased yield in each year respectively, and n is the number of the years of the time interval under consideration.
Instabilities of the yield calculated for each prefecture, are as shown in table 1 and figure 1.
Instability of the yield in the north-eastern provinces and the districts of Inland sea coast is generally large and that in the Kwanto district s small.
It is due to the occurence of unseasonable low temperature end drought respectively that the instability of the yield in the northern part of Japan and the districts of Inland sea coast is larger than the other districts.
That the instability of the yield in the Kwanto district is smaller than that in other districts, may be explained as due to the fact, that sweet potato _??_n the Kwanto district does not only so often suffer great injury by unseasonable low temperature and drought, but also other conditions than weather factors are more favourable for the sweet-potato culture than in other districts.