Hello Steve, I have some challenges with interpretation of the regression coefficient of a mixed model I wonder if you could help. So my PhD study has repeated measured design with 5 time-point: baseline (Time=1) and 2, 3, 4, and 5 (follow-ups) over 3 years. The aim of the study is to compare the effectiveness of two interventions for heroin dependence (MMT and CCT). Below are the SAS syntax of the final model, the outputs of fixed effects and my attempts in doing interpretation in the format of regression model. I wonder if you could have a look and comment on the accuracy of my work. The outcome variable here is number of drug-free days during the previous 30 days (DrugFreeDays) which is treated in the model as a continuous outcome variable (with minimum value of 0 and maximum value of 30). This model has a curvature shape, hence Time*Time in the random statement. Thank you very much for your time. Thu Vuong *Model 12: Re-run the final model (Model 11) in REML; proc mixed method=reml; class Group Time0 IDNumber PolyUseEver; model DrugFreeDays= Group Time PolyUseEver Group*Time PolyUseEver*Time/solution; random Int Time Time*Time/subject=IDNumber Type=UN; repeated Time0/subject=IDNumber Type=ar(1); run; Solution for Fixed Effects Effect Group Poly Use Estimate Standard Error DF t Value Pr > |t| Ever Intercept -2.5439 0.3329 589 -7.64 <.0001 Group CCT 2.9993 0.3669 929 8.18 <.0001 Group MMT 0 . . . . Time 7.4011 0.1571 589 47.11 <.0001 PolyUseEver No -0.9113 0.3636 929 -2.51 0.0124 PolyUseEver Yes 0 . . . . Time*Group CCT -3.6112 0.1737 929 -20.8 <.0001 Time*Group MMT 0 . . . . Time*PolyUseEver No 0.5241 0.1721 929 3.05 0.0024 Time*PolyUseEver Yes 0 . . . . The regression model is written as below: DrugFreeDays = -2.5439 + CCT * 2.9993 + Time * 7.4011 + No PolyUser * (-0.9113) + Time*CCT*(-3.6112) + Time*No PolyUserEver* (0.5241); Time is a continuous variable valued from 1 to 5. The intercept value of -2.5439 is the value of the mean number of drug-free days for MMT participants when Time=0. Therefore, when Time=1 (baseline), the value is (-2.5439) + (7.4011) = 4.8572; For a CCT participant, this value is 4.8572 + 2.9993 = 7.8565 (at baseline); At Time=5, for a CCT participant who was a poly user: Y=-2.5439 + 2.9993 + (4*7.4011) + 0 – (4*3.6112) =-2.5439 + 2.9993 + (29.6) + 0 – (14.44)= 15.61 days At Time=5, for a MMT participant who was a poly user: Y=-2.5439 + (4*7.4011) + 0 + 0 =-2.5439 + (29.6) = 27.06 days ----
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