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From |
Robin Luo <[email protected]> |

To |
[email protected] |

Subject |
st: very confused - how to implement a dynamic lagged variable |

Date |
Mon, 21 Feb 2005 01:50:55 -0800 |

I got really confused by this problem: I want to use a lagged variable in my analysis but instead of lagging based on some pre-existing value, this lag actually comes from certain calculation. My data is a panel data (grouped by "firmid") with a structure like: firmid year age_a age_b weight_a weight_b 100 1960 10 100 1961 11 2 .9 .1 100 1962 12 100 1963 13 10 .7 .3 100 1964 14 100 1965 15 100 1966 16 5 .8 .2 101 1940 1 101 1941 2 10 .6 .4 101 1942 3 The goal is to calculate an adjusted age for each firm-year. In the data, age_a is the natural age of the firm, while age_b comes from certain event (say, an acquisition). Before there is any event, the firm's adjusted age is simply the natural age (age_a). When there is an event, the new adjusted age is the weighted sum of the adjusted age and age_b (weight_a is the weight for adjusted age while weight_b for age_b). When there is no event, the firm's adjusted age simply increase by 1 for each year. I thus wrote the following program: clear input firmid year age_a age_b weight_a weight_b 100 1960 10 . . . 100 1961 11 2 0.9 0.1 100 1962 12 . . . 100 1963 13 10 0.7 0.3 100 1964 14 . . . 100 1965 15 . . . 100 1966 16 5 0.8 0.2 101 1940 1 . . . 101 1941 2 10 0.6 0.4 101 1942 3 . . . end sort firmid year gen age=0 by firmid: replace age=age_a if _n==1 & age_b==. by firmid: replace age=(age_a*weight_a + age_b*weight_b) if _n==1 & age_b~=. by firmid: replace age=(age[_n-1]+1)*weight_a + age_b*weight_b if _n~=1 & age_b~=. by firmid: replace age=(age[_n-1]+1) if _n~=1 & age_b==. The result, however, came quite strange: firmid year age_a age_b weight_a weight_b age 100 1960 10 10 100 1961 11 2 .9 .1 10.1 100 1962 12 11.1 100 1963 13 10 .7 .3 3.7 100 1964 14 4.7 100 1965 15 5.7 100 1966 16 5 .8 .2 1.8 101 1940 1 1 101 1941 2 10 .6 .4 5.2 101 1942 3 6.2 For the 4th and 7th observations (where "year" are 1963 and 1966 respectively), the correct ages should be about 11 ([11.1+1]*0.7+10*0.3) and 6 ([5.7+1]*0.8+5*0.2). Obviously, instead of getting the actual last "age" values, STATA took the very original ones - the generated 0s. I could not figure it out and really appreciate any help on this. Thanks a lot! Robin Luo * * For searches and help try: * http://www.stata.com/support/faqs/res/findit.html * http://www.stata.com/support/statalist/faq * http://www.ats.ucla.edu/stat/stata/

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