With 48 data , the R2 of the model is 0.9733 in 6 months and is 0.8574 in 12 months, 36 and 42 data also can be used for building ARIMA model ,the R2 of them were lower than that of 48 data.

• 用48个数据建模；6个月预测效果的拟合度R2=0.；9733；12个月预测效果的R2=0