Deciding if a covariate is useful or not

Hi,

I am analyzing CBC HB data and I am trying a combination of 3 covariates to improve the model. for each of the 8 models ( 1 without covariates & 7 of the different combinations) I have calculated the aggregate RLH and percentage of levels that are 90% of the times positive or negative during the used iterations. the results are as follows and I am not sure if (Comorb+Calm) would be the best choice or not. Note that the S.E. increases as the number of covariate levels increase. Am I missing more information to decide?.

RLH*100 covariate levels Max. # affected levels/covariate levels (affected intercepts)
No covariates 72.99% Ref. Ref.
Comorbid 73.6% 1 4/1 (-ve & +ve equal) (13)
Importance 73.7% 1 4/1 (-ve & +ve equal) (15)
Calm death 76.3% 4 26/4 (-ve) (9)
Comorb+Impt 74.24% 2 9/2 (-ve) (13)
Comorb+Calm 79.3% 5 31/5 (-ve) (8)
Import+Calm 78.8% 5 30/5 (+ve) (8)
All 3 80.2% 6 25/6 (-ve) (5)

Resolved