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

To |
statalist@hsphsun2.harvard.edu |

Subject |
Re: st: manual weighted average variable in panel data set |

Date |
Fri, 16 Nov 2012 00:56:13 +0000 |

No loops are needed. . l id conc resid_time +--------------------------+ | id conc resid_~e | |--------------------------| 1. | 20059 15.96 380 | 2. | 20059 21.17 100 | 3. | 20059 18.07 480 | 4. | 20060 30 181 | 5. | 20060 16.68 292 | |--------------------------| 6. | 20061 23.78 269 | 7. | 20061 18.07 103 | +--------------------------+ . bysort id : gen sumw = sum(resid_time) . by id : gen sumwc = sum(conc * resid_time) . by id : gen meanc = sumwc[_N] / sumw[_N] . l +-------------------------------------------------------+ | id conc resid_~e sumw sumwc meanc | |-------------------------------------------------------| 1. | 20059 15.96 380 380 6064.8 17.55771 | 2. | 20059 21.17 100 480 8181.8 17.55771 | 3. | 20059 18.07 480 960 16855.4 17.55771 | 4. | 20060 30 181 181 5430 21.77708 | 5. | 20060 16.68 292 473 10300.56 21.77708 | |-------------------------------------------------------| 6. | 20061 23.78 269 269 6396.82 22.19901 | 7. | 20061 18.07 103 372 8258.03 22.19901 | +-------------------------------------------------------+ This code is too long for efficiency, but shows what I believe you want. You can also generate sumwc / sumc if you wish. I didn't try to follow your code, but you're missing how -sum()- does cumulative sums. See also _gwtmean from SSC (David Kantor). (I don't agree with his advice to make it a substitute for Stata's code for -gmean()-.) Nick On Fri, Nov 16, 2012 at 12:33 AM, hind lazrak <hindstata@gmail.com> wrote: > I have a panel data set with the following variables: ID time_resid conc > The repeated observations vary from 1 to 5 for each individual (ID). > Each observation has a time period (time_resid) during which a > pollutant concentration occurs (conc). > Here is an excerpt of the data > > list id conc resid_time counter in 1/7, sepby(id) > > +-------------------------------+ > id conc resid_~e counter > ------------------------------------ > 1. 20059 15.96 380 1 > 2. 20059 21.17 100 2 > 3. 20059 18.07 480 3 > ------------------------------------ > 4. 20060 30 181 1 > 5. 20060 16.68 292 2 > ------------------------------------ > 6. 20061 23.78 269 1 > 7. 20061 18.07 103 2 > +------------------------------------+ > > I need to create a variable using time_resid and conc that computes > the time-weighted average concentration for each ID. > > The steps that I took were to create variable product (equal to > resid_time * conc) and then I have been trying to come up with a loop > that would do the following: > for each person, and each observation compute the time weighted > average concentration =( sum of product / total resid_time up) until > the time at which the observation occurs. > > Here's my code: > *************************************** > bysort id: gen counter = _n > bysort id: gen product= resid_time*conc > > bysort id: gen time = resid_time if _n==1 | counter[_N]==1 > bysort id: gen twa= conc if _n==1 | counter[_N]==1 > qui su counter > forval i=1/`r(max)' { > bysort id: replace product= product+ product[_n-`i'] if _n!=1 > bysort id: replace time= resid_time+ resid_time[_n-`i'] if _n=1 > } > bysort id: gen twa = product/time * * 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/

**Follow-Ups**:**Re: st: manual weighted average variable in panel data set***From:*hind lazrak <hindstata@gmail.com>

**References**:**st: manual weighted average variable in panel data set***From:*hind lazrak <hindstata@gmail.com>

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