Measuring the tax gap is prone to conceptual and practical difficulties. A starting point is that even if you could measure it, you would not want to collect every single dollar of outstanding taxes. You would end up with a lot of your constituents complaining to you that Canada Revenue Agency was being too aggressive. If you say that the tax gap is $100 billion, don't think that the $100 billion is just waiting out there to be pulled in.
The second issue, of course, is that if you could measure it accurately, you could very simply probably tax it.
I think that the most productive thing to do is track what progress Revenue Canada is making. Are they in fact increasing the yield from offshore non-compliance? Are their risk management tools identifying the sectors and the types of taxpayers that are most prone to evasion? That's the way you can see whether or not progress is being made in closing the tax gap.
Some countries have tried to measure the tax gap, and probably the best example is the United Kingdom, which looked at the gap under the VAT. I think what they found was that the absolute figure is far less interesting than a time series, which actually shows you what's happening in terms of whether you're doing the right thing and whether you're closing the gap.
I think you just have to be very careful and not see the tax gap as some sort of silver bullet that's suddenly going to solve an issue.