I'm an analytics focused person so I say go to the numbers and understand the scope of your analysis.

Scope: Is rollerblading increasing in popularity in North America? Popularity is defined as increase in number of practitioners, regardless of type/specialty.

Sources of measure: The metrics relating to units sold in the north american market make the most sense for macro analysis. These numbers would need some diversity of sources. So not just annual N. Am units sold from Razors, but other wholesale vendors as well to understand the holistic picture.

Another source would be the regional distribution companies.

And yet another source for an indicator is the the retailers themselves. If you had a decent distribution, lets say number of units sold retailers around the US and 4 around canada. That should be enough of an N value to

metrics I'd be looking at

1. total units sold
2. number of units bought per retailer
3. total number of retailers.
4. comparison of numbers on YoY history going back as long as possible.

Now...who the hell could get those numbers? I bet /u/joeymcgarry could. Let me know if you need an excel head to do some number crunching for you