Weight and Strength of Evidence

A.6.2.1 For the purpose of classification for carcinogenicity, substances are allocated to one of two categories based on strength of evidence and additional weight of evidence considerations. In certain instances, route-specific classification may be warranted. 

The classification of a substance in Category lA and IB is based on strength of evidence together with weight of evidence considerations (See paragraph A.6.2.5). Such evidence may be derived from ... 

Carbon-14 labelled co-catalysts have been used on a number of occasions to measure the numbers of active sites. Usually, however, osmotically determined number average molecular weights, were not reported. Thus it was not possible on these occasions to demonstrate unequivocally that nc< l which is a prime requirement for meaningful results. Recently, Ayrey and Mazza (80) have examined the titanium trichloride-triethyl aluminium catalysed polymerization of styrene at 60° and have found values of ne 3—10. They also obtained indirect evidence for the formation of poiyethylene-14C during the preparation of the catalyst. Similar observations have been reported previously (92). On the strength of these observations the use of labelled co-catalysts to measure active centres must be regarded as a somewhat suspect procedure. This conclusion is borne out by the recent kinetic work of Coover et al. (81). 

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