linear regression - Trying to get confidence/prediction intervals with `predict.lm` in R, but I keep getting an error regarding my dichotomous variable -
    i have dataset looks this:      time size type 1    22  151    0 2    31   92    0 3    26  175    0 4    35   31    0 5    27  104    0 6     5  277    0 7    17  210    0 8    24  120    0 9     9  290    0 10   21  238    0 11   33  164    1 12   20  272    1 13   16  295    1 14   43   68    1 15   36   85    1 16   26  224    1 17   25  166    1 18   18  305    1 19   35  124    1 20   19  246    1   my aim simple: run regression , confidence/prediction intervals.   i run regression so:   fit.lm1<-lm(time~size+type,data=project3)   i want 95% confidence intervals , 95% prediction intervals mean time  when size  equal 200.  want cis/pis type  = 0 , type  = 1.  code is:   new_val <- data.frame(size= c(200,200),type=c(1,0)) ci<-predict(fit.lm1,newdata=new_val,interval="confidence") pi<-predict(fit.lm1,newdata=new_val,interval="prediction")   i following errors:   error: variable 'type' fitted type "factor" type "numeric"...