Why Prediction Interval is Soooooooooooooooo WIDE?

[#1]

Regression Control Charting Using the Random Coefficient Regression Method 

Dears, 

I was trying to analyze the stability test data.

with SCRIPT which was attached in upper Link.

However, the PI(Prediction interval) is extremely wide as it is seen as following picture.

I have no idea why this happen.

Please be aware that if more month of observation is added, PI range drastically reduces.

i.e. add 2 more mont

[#2 · Accepted solution]

Did you step through the script to see why the interval is so wide? Did you read the reference articles that might discuss the nature of the interval and what to expect?

Did you contact the author of the paper that you cite?

[#3]

It is complicated to tell the story. 

I read the article, but no idea.

I have no experience of reading a script but I did tried but so far,

I finally figured out which part of the script is responsible for a PI but still no idea.

I got your point and I will contact the author.

Thanks.