Hi, newbie here, so please forgive the bad terminology. I started with ten years of historical data. I used Excel to give me a linear trendline and equation. I used that equation to get the forecasted data points for the following ten years. So far so good.

Now I want to get the data points for a 95% confidence interval on this forecast. For the one-step forecast, I know that I just need to go +-2 SDs to get this interval. However, how many SDs should I use for steps 2-10? Is there some table somewhere that will tell me how many SDs to use for a given step forecast and confidence interval?

I know that I could just use 2 SDs for each step of the forecast, but then the interval does not diverge, it just runs parallel to the point estimates. Since the cone of uncertainty should expand over time, the number of SDs should get larger, right?

Now I want to get the data points for a 95% confidence interval on this forecast. For the one-step forecast, I know that I just need to go +-2 SDs to get this interval. However, how many SDs should I use for steps 2-10? Is there some table somewhere that will tell me how many SDs to use for a given step forecast and confidence interval?

I know that I could just use 2 SDs for each step of the forecast, but then the interval does not diverge, it just runs parallel to the point estimates. Since the cone of uncertainty should expand over time, the number of SDs should get larger, right?

Last edited: