[desu_]

I searched for "underground", "black" (market/economy) and "undeclared" in the article, but it yielded no results. I am surprised given that (1) the rise of the underground economy is common knowledge for people knowing a bit about the region and (2) measuring its size is a mainstream academic question.

[rb666]

Same reason that watching pirated movies/shows is not counted in viewership statistics. It's illegal, you don't get credit for it. Taxes exist for a reason.

[desu_]

Viewership statistics of movies do not take into account pirated movies mostly because it's difficult to count. Tax offices might only care about movies sold, but producers regularly try to estimate their losses due to pirate channels, and I am sure that people who measure viewership as a proxy for impact on the popular culture would love to have to be able to count everyone.

The article's author makes some observation on employment volumes in Southern Europe, then decries "the Eurozone's failure to implement meaningful pro-growth reforms". In that context, It makes sense to try to estimate how much of the economic activity has simply been reclassified. The fact that it isn't taxed makes it, once again, more difficult to measure (if that's what you meant by your "Taxes exist for a reason" truism) but not irrelevant.

[justincormack]

Illegal activity is included in GDP figures. Measuring it is hard though.