Averaging Panel Data in SAS
===

I have panel data set that looks like this
    

    ID    Usage     month    
    1234    2        -2  
    1234    4        -1
    1234    3         1
    1234    2         2
    2345    5        -2
    2345    6        -1
    2345    3         1
    2345    6         2

Obviously there are more ID variables and usage data, but this is the general form. I want to average the usage data when the month column is negative, and when it is positive for each ID. In other words for each unique ID, average the usage for negative months and for positive months. My goal is to get something like this.
       

    ID   avg_usage_neg   avg_usage_pos
    1234     3                  2.5
    2345     5.5                4.5