Table 2.

Cross-validation results.

Stratum	Cross validation	Kriging	IDW	Thin plate spline	Regular spline function
Z0	Average error	0.845	1.177	0.979	0.951
	Root mean square	1.144	1.600	1.251	1.286
Z1	Average error	1.591	2.203	2.584	2.225
	Root mean square	2.132	2.852	3.124	2.809
Z2	Average error	–	3.029	3.136	2.859
	Root mean square	–	3.805	4.110	3.711
Z3	Average error	–	5.442	5.943	5.646
	Root mean square	–	6.523	7.131	6.744
Z4	Average error	–	5.714	9.198	6.419
	Root mean square	–	6.783	10.422	6.657
Z5	Average error	–	5.715	6.328	4.585
	Root mean square	–	5.581	7.503	5.766