Bias RMSE Nominal 95%
Coverage: non-
prob. weights
constant
Nominal 95%
Coverage: non-
prob. weights
as random
Mean of
sample
means
of W
Prob. sample only 0.011 0.201 89 0.548
Prob. sample and non-prob. 0.041 0.201 12 1.253††
sample combined*
Prob. sample and non- –0.061 0.157 93 97 0.360††
prob. sample combined** –0.358 0.477 60 85 –0.137††
Linear in W –0.071 0.154 90 94 0.357††
Quadratic in W –0.336 0.424 65 78 –0.096††
Linear in W and X
Quadratic in W, linear in X