Collapsing Data Issue

[Professor Stata Data]

I tried this:

collapse (mean) mpg_all ///
, by(state_code.year.month)

Received this error:

factor-variable and time-series operators not allowed
(error in option by())

What does this mean, Please Help!

[Joseph Coveney]

It means

state_code.year.month → state_code year month

[Nick Cox]

https://www.statalist.org/forums/help#realnames

6. Why are real names preferred?

You are asked to post on Statalist using your full real name, including given name(s) and a family name, such as "Ronald Fisher" or "Gertrude M. Cox". Giving full names is one of the ways in which we show respect for others and is a long tradition on Statalist. It is also much easier to get to know people when real names are used.

If you overlook this on first registration, it is easy to fix. Click on “Contact us” located at the bottom right-hand corner of every page.