st: Filling data using Ipolate when data used are constrained

["Pavlos C. Symeou" <p.symeou@jbs.cam.ac.uk>]

Dear Stata-listers,

I would greatly appreciate you help with this.

I have data for a number of countries over a series of years. I want to 
fill the missing values of my variables using the -ipolate- method; yet, 
I want the -ipolate- command to be applied separately for each country 
in a manner that the data involved in the estimation are bounded at the 
bottom by the earliest value of each variable. An illustration of my 
data follows.

In the example here, I would like to fill the missing values for 
mobile_users and internet_users. For mobile_users there are zero users 
until 1991 and there is only one missing value for 2004. For 
internet-users the earliest non-missing value is zero in 1990, then a 
few missing values follow until 1996 when non-missing values appear and 
then there is a new gap in the data for 2001 and 2002. For this variable 
I want: a) the variable to take zero values for earlier years than 1990 
and b) for years after 1990 I want the missing values to be filled with 
the -ipolate- command which however uses data between 1990 and 2004 
inclusive.

country    year    fixed_users    mobile_users    internet_users    
population2
Andorra    1980    6819.353    0                                        
                   33
Andorra    1981    9243.998    0                                        
                   37
Andorra    1982    10744.91    0                                        
                   39
Andorra    1983    12396.58    0                                        
                   41
Andorra    1984    13065.76    0                                        
                   42
Andorra    1985    14000          0                                    
                       44
Andorra    1986    15761.06    0                                        
                   46
Andorra    1987    17229.59    0                                        
                   48
Andorra    1988    18415.59    0                                        
                   50
Andorra    1989    19701.27    0                                        
                   51
Andorra    1990    21915.5      0                            0        
                     53
Andorra    1991    24329.42    0                                        
                   57
Andorra    1992    26224.22    798.0653                                
           60
Andorra    1993    27601.18    804.5187                                
           63
Andorra    1994    28608.47    790.8131                                
           65
Andorra    1995    28042.35    2658.823                                
           64
Andorra    1996    27911.21    4946.93        901.4084                
    64
Andorra    1997    28472.68    7672.845     1780.656                     66
Andorra    1998    28992.48    12422.96     3960                      
      66
Andorra    1999    29529.67    18100.73     4393.38                       66
Andorra    2000    28951.15    19921           5923.077                
   66
Andorra    2001    29164.27    24873.94                                
          67.5
Andorra    2002    29385.59    27591.59                                
          69
Andorra    2003    29448.4      43449.6         8413.948                
  70.5
Andorra    2004    37654.93                           11820.89       
            72

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