http://www.answers.com/unionization&r=67
unionisation, v.tr.
1. To organize into a labor union.
2. To cause to join a labor union.
v.intr.
To organize or join a labor union.

Englesko-srpski recnik, S. Koljevic, I. Djuric-Paunovic
okupljanje/udruzivanje u sindikat, organizovanje sindikata

Sto se "unionisation rates" tice, na osnovu clanka koji sam nasla na http://www.abs.gov.au/ausstats/[email protected]/0/592d2f759d9d38a9ca... i koji kaze:
"This article presents information on trade union membership focussing on a number of socio-demographic and labour force characteristics of trade union members. In this article, the trade union membership rate (also referred to as the unionisation rate) is defined as the number of trade union members expressed as a percentage of the number of employees.",
to bi bila stopa udruzenosti u sindikat, odnosno procenat radnika koji su ukljuceni u neki sindikat, a to potvrdjuje i ovaj clanak:
http://www.eiro.eurofound.eu.int/2004/07/feature/se0407103f....

Hope it helps.

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Note added at 29 mins (2006-11-05 13:24:58 GMT)
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Ups! Dok sam ja pisala odgovor i istrazivala clanke na netu, kolege su vec dosle na slicnu ideju. Sorry sto se ponavljam.
B.