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From |
"Joseph Coveney" <[email protected]> |

To |
<[email protected]> |

Subject |
st: Re: calculation of area under curve with stata |

Date |
Mon, 14 Oct 2013 19:01:52 +0900 |

Elmir Omerovic wrote: I am analyzing the blood pressure (BP) levels from the three different groups of animals. BP was measured repeatedly in every animal at 10 different time points within 90 minutes from the start of the experiment. The animals were exposed to 3 different drugs (each group n=8). I understand that one way to compare the effect of the drugs on blood pressure within the whole experimental period would be to calculate the area under the curve and to use this value in a linear mixed model. I would be very grateful if someone could help me with step-by-step instructions how to calculate AUC in stata. -------------------------------------------------------------------------------- You'd probably want to use -integ-, and so you can take a look at the help file and entry in the user's manual for that command. If your first observation is a pretreatment (baseline) value, then you could consider subtracting it from the succeeding measurements and integrating the change scores, or you could use the first value as a covariate in the model and integrate only posttreatment measurements. After integration, you'll have only a single value for each animal, and so you won't be using a linear mixed model. (I've seen this advocated as a way to avoid mixed models.) Joseph Coveney . clear * . set more off . set seed `=date("2013-10-14", "YMD")' . quietly set obs `=3 * 8' . generate grp = mod(_n, 3) . generate double u_i = rnormal() . generate byte pid = _n . forvalues i = 0/9 { 2. generate double mbp`i' = 100 - /// > grp * (5 - abs(5 - `i')) / 2 + /// > 5 * rnormal() 3. } . quietly reshape long mbp, i(pid) j(tim) . . * . * Begin here . * . sort pid tim . quietly generate double ibp = . . tempvar tmpvar0 . quietly levelsof pid, local(pids) . foreach pid of local pids { 2. quietly integ mbp tim if pid == `pid', generate(`tmpvar0') 3. quietly replace ibp = `tmpvar0' if pid == `pid' 4. drop `tmpvar0' 5. } . . // list grp pid tim mpb ibp, noobs sepby(pid) . quietly by pid: keep if _n == _N . anova ibp grp Number of obs = 24 R-squared = 0.2714 Root MSE = 16.3467 Adj R-squared = 0.2020 Source | Partial SS df MS F Prob > F -----------+---------------------------------------------------- Model | 2090.0102 2 1045.0051 3.91 0.0360 | grp | 2090.0102 2 1045.0051 3.91 0.0360 | Residual | 5611.47898 21 267.213285 -----------+---------------------------------------------------- Total | 7701.48918 23 334.847356 . . exit end of do-file * * 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/

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