Trade unions bargain with employers to improve pay, conditions, and decision-making in workplaces; higher rates of union density within an industry or country will generally indicate higher levels of trade union bargaining power, lower rates of density will indicate less bargaining power.
[1] The causes of higher or lower union membership are widely debated.
In the five largest states, California has 15.9% union membership, Texas 4.5%, Florida 6.8%, New York 24.7% (the highest in the country), and Illinois had 15.2%.
[8][9] In France while the overall union density is 8%, in companies over 50 employees this level reaches 43%.
US Bureau of Labor in 2010 notes a difference of median income of 200 dollars between union-members (917), and non-union members (717) without indicating if higher salaries link to more unionisation, or the reverse or in mutuality.