National Income represents the total value of goods and services produced in an economy in a given period. The three main methods capture this value from different perspectives: Production (Value Added) focuses on the value added at each stage of production; Income focuses on the incomes earned by factors of production (wages, rent, interest, profit); Expenditure focuses on the total spending on final goods and services. Cost and accounting principles are used within firms but are not a method to aggregate economic activity at the national level.