My understanding is that in conducting her 1999 report on the underground economy, the Auditor General reviewed all of the studies over about a 17-year time span. In reviewing all those studies, she found the estimates, as the commissioner mentioned, ranging anywhere from around 3% to around 20%. It was determined that these estimates varied widely because of the different definitions of the underground economy, the different methodologies they used, and the different underlying assumptions they employed in doing their studies.
In their review, they estimated that a reasonable range--and these are the words, if I remember correctly, from the report itself--was 4.2% to 4.5%, which you correctly noted was about $12 billion. About $7 billion of that was federal and about $5 billion was provincial. They also noted that this was similar to a Statistics Canada study that was done just prior to that, which I believe had come up with an estimate of about 4.2%.