Selection of mixed-effects parameters in a variable–exponent taper equation for birch trees in northwestern Spain

Table 3 Fixed-effects parameter estimates, approximate significance contrasts, and variance components of Eq. 8 fitted by the restricted maximum likelihood procedure (REML)

Parameter	Estimate	Approx. std. error	Approx. t value	Approx. prob. > \|t\|
a ₀	0.9652	0.0537	17.96	<0.001
a ₁	0.9421	0.0198	47.53	<0.001
a ₂	0.08482	0.0266	3.19	0.001
b ₀	4.094	0.581	7.04	<0.001
b ₁	−0.3939	0.138	−2.85	0.004
b ₂	−0.4112	0.129	−3.18	0.001
b ₃	−4.153	0.660	−6.29	<0.001
b ₄	2.813	0.628	4.48	<0.001
b ₅	0.05345	0.00492	10.87	<0.001
b ₆	0.3332	0.0847	3.93	<0.001
Variance component	Estimate	Approx. std. error	Approx. Z-value	Approx. prob. > \|Z\|
var(u _k1)	0.0003373	0.00004	7.50	<0.001
var(u _k2)	0.007931	0.0011	7.47	<0.001
cov(u _k1, u _k2)	0.001166	0.0002	6.17	<0.001
σ ² (error variance)	1.837	0.120	15.37	<0.001

ISSN: 1297-966X