How is the pure union wage advantage defined? If in a given labor market the wage rate would be $16 without a union and $20 with a union, then what is the pure union wage advantage? Explain how, and in what direction, each of the following might cause the measured union wage advantage to vary from the pure  advantage:

(a) The spillover effect,

(b) The threat effect,

(c) The product market effect,

(d) The superior worker effect.