which categorical predictor should be used which reference group to use for each model why not pick the same reference group for each model what group is considered the reference group for each regression model with a non ordinal predictor which group is considered to be the reference for each model which group should be the reference group for each regression model? which group of substrates should be the reference group for each model? what is the reference group for a categorical predictor which group to use as reference which group should be chosen as reference for each model? which group to select for a regression which type of variable is a reference to a categorical predictor? what is the reference group on each categorical predictor which group should be the reference in a regression which group of substrate1 would you suggest me to choose as the reference group for each model? which group would you suggest me to choose as the reference group for each model? what is the reference group in a categorical model which group should i choose for each type of regression which categorical predictor can be used in a regression which type of group should you select as the reference group for each model?