# R ARIMA Why forecast does have constant variance?

Hi I am new to time series and I got this problem. At first I created a simple seasonal ARIMA model using auto.arima and forecast library.

Then when I tried to make prediction using this model, variance always remained constant. It shouldnt be like that, when you look at train data, variance should be growing. Because of growing trend. (I tried this on other datasets and result was the same)

What should I do? How to change this model, to get rid of constant variance? :(

Thanks.

Code:  antib = a10 model = auto.arima(antib, stationary = FALSE) prediction = forecast(model, h = 150) autoplot(prediction)

• How do you conclude that forecast variance is constant from looking at that plot? It quite obviously is not! Do you know what those blue shaded areas represent? Jun 20 '18 at 12:37
• Well I assume that represent 80%,95% confidence interval. But shouldn t be the line(mean of prediction) wider(verticaly)? Jun 20 '18 at 12:42
• The model you built is additive, not multiplicative, so the line itself won't vary. But the forecasts are clearly not constant variance, as the widening confidence intervals show. Try using a non-arima model, for example, ets. That allows for multiplicative components, which is what it seems you want (and would be better in this case.) Jun 20 '18 at 12:56
• Thanks very much, So I m gonna look at ETS(never heard about it). Is it wise/possible to use ETS with multiple predictors as input variables? Jun 20 '18 at 13:12
• An alternative to ETS is to set lambda=0 in the call to auto.arima. Jun 20 '18 at 14:20