Dear Steve, thanks for you reply. I may have misunderstood your reply or maybe not phrased my question clearly. For that I apologise. The idea is to fit a model where V1-3 can fully explain the outcome variable so as far as my thinking goes, a+b+c = 1 is a requirement of the model. I do still get the same estimates for a, b and c if I replace "c = 1-a-b" with e.g "a = 1-b-c". Where is the flaw in my logic/thinking? Anyway, here is two examples of my code. The proc model I am fairly confident in but I am not sure I am adding the random effect the right way in proc nlmixed. title 'NIRO, arterial, venous, and skin contribution, without intercept'; proc model data=final; ods output ParameterEstimates=NIROnoint; bounds a > 0, a < 1, b > 0, b < 1; parameters a = 0.50 b = 0.20; c = 1-a-b; niro_toi = a*sao2 + b*sjvo2 + c*moor_so2; fit niro_toi; estimate 'Arterial' a, 'Venous' b, 'Skin' c; run; title 'NIRO, random effects model, arterial, venous, and skin contribution, without intercept'; proc nlmixed data=final ; bounds a > 0, b > 0, a < 1, b < 1; parameters a = .5 b = .25 sd1 = 110 sd2 = 0.03; c = 1-a-b; pred = (a*sao2 + b*sjvo2 + c*moor_so2)*r; model niro_toi ~ normal(pred,sd1); random r ~ normal(1,sd2) subject=subject; estimate 'Arterial' a; estimate 'Venous' b; estimate 'Skin' c; run;
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