Complex sentence: In 2010, the percentage of workers belonging to a union in the United States (or total labor union "density") was 11.4%, compared to 18.6% in Germany, 27.5% in Canada, and 70% in Finland.
Split into simpler sentences: In 2010, the percentage of workers belonging to a labor union (or total labor union "density") was 11.4% in the United States. Compared to 18.6% in Germany, 27.5% in Canada, and 70% in Finland.