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From |
Nick Cox <njcoxstata@gmail.com> |

To |
statalist@hsphsun2.harvard.edu |

Subject |
Re: st: identifying change in cell values |

Date |
Fri, 15 Mar 2013 11:52:03 +0000 |

The -tag()- function [NB] of the -egen- command does not have the role of detecting _new_ values. Its role is to tag or flag precisely one of any group of observations that is identical on the variables specified. If you look at the source code with . viewsource _gtag.ado you can see that in practice the function finds the first such occurrence. That was originally (Cox 1999), and is now, just an arbitrary choice. That's by the way. STB-50 dm70 . . . . . . . . . . . . . . . . Extensions to generate, extended (help egenodd if installed) . . . . . . . . . . . . . . . . N. J. Cox 7/99 pp.9--17; STB Reprints Vol 9, pp.34--45 24 additional egen functions presented; includes various string, data management, and statistical functions; many of the egen functions added to Stata 7 I suspect from the previous thread that Adeel opened http://www.stata.com/statalist/archive/2013-03/msg00424.html that his interest in panel data. If so, the principles of identifying spells apply here. In http://www.stata.com/statalist/archive/2013-03/msg00446.html Rebecca Pope directed Adeel to -tsspell- (SSC). The corresponding principles of identifying spells were discussed in SJ-7-2 dm0029 . . . . . . . . . . . . . . Speaking Stata: Identifying spells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . N. J. Cox Q2/07 SJ 7(2):249--265 (no commands) shows how to handle spells with complete control over spell specification which is accessible in .pdf form at http://www.stata-journal.com/article.html?article=dm0029 Note that the article does not mention -tsspell-. The explanation is mundane. I wrote -tsspell- (the help file flags people who contributed to the project, most notably Richard Goldstein) and then later decided to write about the ideas. The original intention was to follow the ideas part with a section on -tsspell- that explained one canned implementation, but the paper was already long enough before I ever got to writing that section. I doubt that there will ever be a sequel on -tsspell-, but I should add a reference to the 2007 paper to the help of -tsspell-. All that aside, a basic way to identify a changed value in panel data is just bysort panelid (time) : gen different = thisvar != thisvar[_n-1] with the proviso that a missing value for -thisvar[1]- will not be flagged as different from -thisvar[0]-, so bysort panelid (time) : gen different = thisvar != thisvar[_n-1] | (_n == 1) might be preferred. Again, for a much fuller discussion, please see my 2007 paper. It is full of little things that are elementary once you have worked them out. Nick On Fri, Mar 15, 2013 at 1:53 AM, Adeel Yousaf <townplanneradeel@gmail.com> wrote: > I applied the 'tag' command that is detecting new values, how can i > detect change in the value of 'coded' either repeating or new. > > egen tagcoded = tag( coded) > > > coded tagcoded > 24 1 > 24 0 > 28 1 > 27 1 > 27 0 > 27 0 > 27 0 > 27 0 > 28 0 > 22 1 > 22 0 > 24 0 * * For searches and help try: * http://www.stata.com/help.cgi?search * http://www.stata.com/support/faqs/resources/statalist-faq/ * http://www.ats.ucla.edu/stat/stata/

**References**:**st: identifying change in cell values***From:*Adeel Yousaf <townplanneradeel@gmail.com>

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