Lamb feeding trial designed as follows:
*Lambs placed into individual pens (experimental unit = lamb ID) and remained in the pen throughout the trial
*Lambs fed for 84 day, using 2 feeding periods (diet changed going into Period 2). Period 1 (days 0 to 28); Period 2 (days 29 to 84)
*On day 0 and multiple times during each period (e.g., d 0, 14, 28; 29, 50, & 84), various things are measured; e.g., body weight (BW), BW gain...
Questions:
1. Can data be analyzed by Period without worrying about/checking for a trt x period interaction (evaluate by period even if interaction is not signif.)? I like this method because of the differences we see at times, where 1 trt does poorly during Period 1 but then "rebounds" in Period 2 and catches up to the others.
PROC MIXED; by Period;
CLASS ID TRT DAY;
MODEL BW = TRT day TRT*day;
REPEATED DAY / SUBJECT=ID TYPE = XXX;
2. Or, can I just analyze without including period anywhere in the model; just use day? Will the meaning of the results be confounding if I examine effects over different Periods? For example, lambs with less BW gain early in the feeding period may go through accelerated gain in Period 2 and that greater ADG may not be because of the feeding period but because of the previous ADG.
PROC MIXED;
CLASS TRT DAY ID;
MODEL BW = TRT DAY TRTxDay / XXX;
REPEATED DAY/SUBJECT=ID TYPE = XXX
Thanks
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