Canadian government's own study shows voluntary census fails compared to a mandatory census

Government study reveals significant errors in voluntary census - The Globe and Mail
No surprise here.
Excerpt:
A June, 2010, internal study obtained by The Globe and Mail under the
access-to-information law offers an inside look at how new census-taking
rules could skew data in a range of areas from housing to demographics.
Statscan researchers found the voluntary approach produced less accurate
results – a problem that was especially significant in small population
groups, according to outside statistics advisers who reviewed the
report for The Globe.