What role did the doughboys play during WWI?

What would lead a historian to consider two events to have a relationship of correlation rather than causation?

Were african-americans allowed to serve in the military in wwi?

Why might collective bargaining fail, causing a union to go on strike? a the union agrees with the boss about wages. b the boss compromises about work hours. c the factory goes out of business, causing the workers to lose their jobs. d the boss refuses to address dangerous working conditions in the factories.