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From |
"Rajesh Tharyan" <R.Tharyan@exeter.ac.uk> |

To |
<statalist@hsphsun2.harvard.edu> |

Subject |
RE: st: keeping dates which are n periods apart |

Date |
Mon, 19 May 2008 18:02:53 +0100 |

Hi nick, Thanks very much for that, what you suggest is exactly what I needed. I think it must been my wording of what I wanted to achieve that prompted the complicated approach. This was the original statement of the problem... In an earlier post 1. Start with the first date. 2. See if the next date is more than 730 days from it. if its less than 73o days away from the first date drop it. 3. If it is more than 730 days away keep it and 4. Now this date becomes the reference date and the difference between this date and the next is calculated and so on However, Scotts approach to use mod and your final suggestion was all that was needed. Thank you Regards Rajesh -----Original Message----- From: owner-statalist@hsphsun2.harvard.edu [mailto:owner-statalist@hsphsun2.harvard.edu] On Behalf Of n j cox Sent: 19 May 2008 15:37 To: statalist@hsphsun2.harvard.edu Subject: Re: st: keeping dates which are n periods apart The word description of your problem -- concisely given in your title -- suggests a simple solution based on Scott Merryman's excellent suggestion of -mod(,)- as a basis for your problem. keep if mod(year, 3) == mod(1987, 3) gives the solution that you specify for the first example. The choice of 1987 reflects the fact that 1987 is the start of your first example. Note that this 1. Does not entail any attention to the panel structure of your data. 2. Ensures that the same years are selected for all panels when available. 3. Can be transferred to other periods by changing 3 to what is desired. 4. Can be modified easily for aspects of the problem not specified. For example, you could calculate mod(year, 3) and choose a start based on what kept most observations. Or, very irregular panels might be sampled by bysort id (year) : keep if mod(year, 3) == mod(year[1], 3) which for the second example below (different from the first) selects 1986(3)1995. Conversely, I don't understand why much more complicated solutions have been offered for this kind of problem. What am I missing? Nick n.j.cox@durham.ac.uk Rajesh Tharyan I have this ( a subset of the original dataset, the original dataset has about 6000 ids with an average of 6 years each) id year 4 1987 4 1988 4 1989 4 1990 4 1992 4 1993 4 1994 9 1987 9 1988 9 1989 9 1990 9 1992 9 1993 9 1994 I need to keep years if they are more than 2 years apart by company. In an earlier post http://www.stata.com/statalist/archive/2006-02/msg00952.html David Harrison suggested the following which works fine with one id local refdate = year[1] gen byte dropflag = 0 forvalues i = 2/`=_N' { if year[`i']-`refdate'<=2 { replace dropflag = 1 in `i' } else { local refdate = year[`i'] } } drop if dropflag drop dropflag However I am not able to do it by id. what I would want is this.. id year 4 1987 4 1990 4 1993 9 1987 9 1990 9 1993 I tried using the above code with levelsof and foreach as shown below but no luck. ******************* input id year 4 1987 4 1988 4 1989 4 1990 4 1992 4 1993 4 1994 9 1986 9 1987 9 1988 9 1989 9 1990 9 1992 9 1993 9 1994 9 1995 9 1996 9 1997 end levelsof id, local(levels) foreach l of local levels { qui sum year local refdate = year[1] gen byte dropflag = 0 forvalues i = 2/`=_N' { if year[`i']-`refdate'<=2 { replace dropflag = 1 in `i' } else { local refdate = year[`i'] } } drop if dropflag drop dropflag } list, table clean noobs ******************* which gives me id year 4 1987 4 1990 4 1993 9 1996 I tried a variation of the above as **Snip** levelsof id, local(levels) foreach l of local levels { qui sum year if id==`l' local refdate = r(min) local m= r(N) gen byte dropflag = 0 forvalues i = 2/`m'{ if year[`i']-`refdate'<=2 { replace dropflag = 1 in `i' } else { local refdate = year[`i'] } } drop if dropflag & id==`l' drop dropflag } **Snip** That gives me id year 4 1987 4 1990 4 1993 9 1995 9 1996 9 1997 Any pointers much appreciated * * 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/ * * 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/

**References**:**Re: st: keeping dates which are n periods apart***From:*n j cox <n.j.cox@durham.ac.uk>

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