Final answer:
Primary factors affecting changes in labor supply include the wage rate, which causes a movement along the supply curve, and other aspects like job desirability and government training policies that shift the supply curve.
Step-by-step explanation:
The primary reasons for relative changes in labor costs can encompass a variety of factors. These include changes in hours worked, total output, inflation rates, exchange rates, and the level of education. However, based on the provided text, the factors that directly cause the supply of labor to increase or decrease are changes in the wage rate (the price of labor), shifting the supply curve, changes in job desirability, and government policy related to labor market training. It is not specifically mentioned in the text that hours worked, total output, inflation rates, or exchange rates are factors leading to a supply curve shift, but they can influence the supply and demand in different contexts.