Dear all,

Assume that you have the following problem. You have patent citations for firms in different years. The questions is to calculate the half-life of citations. That is, how many years will be needed until a firm gets 50% of the citations from a specific year. I have prepared the following simplistic example:

Code:
 input id    year    value    half_value    time_until_half
1    1990    50    25    3
1    1991    2    1    1
1    1992    3    1.5    1
1    1993    25    12.5    1
1    1994    15    7.5    3
1    1995    5    2.5    1
1    1996    1    0.5    1
1    1997    1    0.5    1
1    1998    3    .    .
2    1990    8    4    2
2    1991    3    1.5    2
2    1992    1    0.5    1
2    1993    2    1    1
2    1994    12    6    4
2    1995    1    0.5    1
2    1996    2    1    1
2    1997    1    0.5    1
2    1998    3    .    .
end
In this example, we have two firms "1" and "2". Firm "1" starts with 50 citations in year 1990. Based on the citation data for the following years, one can find that 3 years will be needed until the half-value (25) is reached. In the same manner we can calculate half-life for the other citations. For example, in year 1991 firm "1" has 2 citation (half-value is 1), so it just needs one year to reach that value (in year 1993 it has 3 citations).

How would one calculate half-life in the way described above in Stata?

Thanks a lot