I think the evidence is in fact that people such as these, who are somewhat marginalized, would be less likely to fill out the census form. I would agree with you that they will therefore be underrepresented in a voluntary census. When it is mandatory, I think they realize, despite the struggle they have and despite their reluctance, that it is important, and as a Canadian citizen they feel they want to contribute to it. That signal of “mandatory” is sent to them, and as a result they fill it out, despite their reluctance. Without the mandatory aspect, they may feel, well, let someone else fill it out.
