Re: Truncated normal distribution for continuous covariate with known mean and standard error

Dear Åsa, Thank you very much for your suggestions. I have tried both approaches and the one you prefer (using CALLSIMETA) is more intuitive I also think. It seems to work fine. In the code you suggest (using CALL RANDOM) a 70 kg weight for all subjects is attained. I think this is due to R = 0 - when I remove it, I start getting different weight values that are within the truncation limits. Lastly, I am very interested in hearing more regarding your end comment on correlation of the simulated covariates. Could you briefly specify what you mean/how to implement this (i.e. how to preserve the same correlation patern as in the original data set). I am simulating distributions for both continuous (e.g. weight, age) and categorical (e.g. SEX, SNPs) covariates. It is for the categorical covariates that I use separate CALL RANDOM blocks while drawing from UNIFORM distributions. Best regards, Yassine 2015-12-08 14:22 GMT+01:00 Johansson, Åsa <asa.johansson om>: > Dear Yassine, > > > > I prefer the second option, where you simulate new ETAs until they fulfil l > your requirements. THETA(1) is the mean value of WT while ETA(1) is a > random variable with variance OMEGA. You should just add the known mean > value of WT (e.g. 70) as the initial estimate of THETA(1) and the known > variance of weight (stdev^2; e.g. 100) as the initial estimate of OMEGA f or > ETA(1). The simulations will always be based on your initial estimates so > there is no need to specify the actual values (i.e. mean 70 and standard > deviation 10) directly in the code. By the way, I think you might need to > add a .AND.NEWIND.NE.2 to the IF-statement (IF(ICALL.EQ.4.AND.NEWIND.NE.2 ). > > > > If you anyhow want to use the first option (simulate random numbers from a > normal distribution), the CALL RANDOM(5, R) needs to be specified in the > DOWHILE-loop. You want to simulate a new random variable if the first one > doesn’t fulfill your requirements. This code could look something like this: > > > > IF(ICALL.EQ.4.AND.NEWIND.NE.2) THEN > > R = 0 > > WT p+10*R > > DOWHILE (WT.LT.50.OR.WT.GT.100) > > CALL RANDOM(5,R) > > ENDDO > > ENDIF > > IF(ICALL.EQ.4) WTp+10*R > > > > If the simulated weight is outside the specified limits (i.e. less than 5 0 > or greater than 100), a new random value will be simulated until the weig ht > is between these limits. I have not tried the code myself but I’m pretty > sure it should work. > > > > You write in your question that you have 4 different covariates which you > want to simulate. Be aware that the simulated covariates need to be > correlated in the same way as your original data to give valid results. > > > > Kind regards, > > Åsa > > > > > > *From:* owner-nmusers ] > *On Behalf Of *Yassine Kamal Lyauk > *Sent:* den 8 december 2015 13:14 > *To:* nmusers > *Subject:* [NMusers] Truncated normal distribution for continuous > covariate with known mean and standard error > > > > Dear NMUsers, > > > > I would like to implement a truncated normal distribution for weight for > use in a simulation, with a pre-specified mean and standard deviation. > > > > What works so far is this (w.o. truncation, which I found in the NONMEM > user guides): > > > > $PK > > IF (ICALL.EQ.4.AND.NEWIND.NE.2) THEN > > CALL RANDOM(5,R) > > ENDIF > > IF (ICALL.EQ.4) WTp+10*R; mean of 70 with SD 10 > > > > $SIM (12345) (...) (...) (...) (54676 NORMAL); I have a total of five > random number generators i.e. 4 different covariates, which I want to > simulate distributions for. > > > > Adding a DOWHILE loop does not seem to work (either I get error messages > or NONMEM stagnates after performing the sims and never finishes) - I thi nk > it's a coding issue on my part, in regard to the IF THEN ELSE statements > and DOWHILE statement: > > > > IF (ICALL.EQ.4.AND.NEWIND.NE.2) THEN > > CALL RANDOM(5,R) > > ENDIF > > IF (ICALL.EQ.4) WTp+10*R; > > DOWHILE (WT.GT.50.OR.WT.LT.100); truncation limits of 50 and 100 > > ENDDO > > > > Other ways to generate a normal truncated distribution can be coded as > (from Metrum Institute course 210): > > > > $PK > > IF (ICALL.EQ.4) THEN > > WT=THETA(1) + ETA(1) > > DOWHILE (WT.LT.20.OR.WT.GT.100) > > CALL SIMETA(ETA) > > WT=THETA(1) + ETA(1) > > ENDDO > > ENDIF > > > > $SIM (2345 NEW) > > > > Yet, I am unsure of how to incorporate my known mean and standard > deviation using this approach. > > > > Hope you can help. Thanks in advance. > > > > Best regards, > > > > Yassine > > > > > > > > > > > > > > ------------------------------ > > *Confidentiality Notice: *This message is private and may contain > confidential and proprietary information. If you have received this messa ge > in error, please notify us and remove it from your system and note that y ou > must not copy, distribute or take any action in reliance on it. Any > unauthorized use or disclosure of the contents of this message is not > permitted and may be unlawful. > > -- Best regards / Med venlig hilsen Yassine Kamal Lyauk