**Table 3:** Computed t-values from the comparison of the contrast for different years with respect to the activity minimum contrast (1996, otherwise indicated), for the $B/\mu$ bins shown in Figs. 3 and 4.
$B/\mu$	1997	1998	1999	2000	2001
< 60 G	0.05	0.21	1.18	0.40	0.21
60 - 90 G	0.07	0.18	1.24	0.52	0.11
90 - 130 G	0.07	0.10	1.24	0.59	0.05
130 - 200 G	0.03	0.22	0.99	0.52	0.10
200 - 300 G	0.43	1.02	0.02	0.19	0.77
300 - 400 G $^{{\rm *}}$	-	1.01	0.30	0.27	0.81
400 - 500 G $^{{\rm **}}$	-	-	0.12	0.20	0.30
500 - 600 G $^{{\rm **}}$	-	-	0.63	0.40	0.09

$^{\rm star}$ Comparison with respect to 1997; $^{\rm star*}$ Comparison with respect to 1998.

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