It is standard practice that a positive reversal test is claimed on the basis of inability to reject the hypothesis that two distributions share a common mean direction, and thus the claim of a positive reversal test is in fact often based on a lack of information. This is unsatisfactory. Therefore it is suggested that positive reversal tests should be classified according to the amount of information that was available for the test. This amount of information is readily indicated by the critical angle (e.g., at the 95 per cent confidence level) between the two sample mean directions at which the hypothesis of common mean direction for the distributions would be rejected. It is recommended that 5", 10" and 20" be used as the breakpoints in the classification.
S U M M A R Y Previous techniques for judging the significance of a palaeomagnetic fold test are either invalid or insufficiently flexible. It is shown that under appropriate circumstances an isolated-observation test may be added to the range of statistical tests used to judge a fold test. A powerful new test is developed based on a test statistic that is sensitive to correlation between the distribution of site-mean directions about the overall mean direction and the tectonic corrections. This test is sufficiently flexible that it should cover most circumstances.
A recent model for polarity reversals suggested that reversals involve critical interactions between the primary (dipole) and secondary (quadrupole) dynamo families. This model predicted that the relative secondary family contribution to the field would be smaller when the reversal rate is low than when the reversal rate is high. In particular, therefore, it was predicted that the contribution from the secondary family would have been low during the Cretaceous Normal Superchron. This prediction is tested using model G for the paleosecular variation of lavas to estimate the relative contributions of the two families back through time. It is found that the data support the prediction. It is also found that a decrease in the contribution from the secondary family is associated with an increase in the contribution from the primary family, and vice versa.
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