Introduction:
The method of calculating National Income by the product method is also known as the value added method. It is one of the three primary methods used to measure the total output and income of a country's economy. The other two methods are the income method and the expenditure method. The value-added method focuses on the value added at each stage of production and distribution to avoid double-counting.

Explanation:
The value-added method calculates the National Income by summing up the value added at each stage of production in an economy. It considers the value added by each producer or firm in the production process. The value added is the difference between the value of output and the value of intermediate goods used in the production.

Key Steps:
The calculation of National Income by the value-added method involves the following steps:

1. Identifying the different stages of production: The first step is to identify the various stages of production in the economy. This includes primary, secondary, and tertiary sectors.

2. Calculating the value added: In each stage of production, the value added is calculated by subtracting the value of intermediate goods and services from the value of output. Intermediate goods are those goods and services that are used in the production process and are not sold as final goods.

3. Avoiding double-counting: The value added at each stage of production is calculated separately to avoid double-counting. This means that only the value added by each firm or producer is included in the calculation, not the total value of the output.

4. Summing up the value added: The value added at each stage of production is then summed up to obtain the total value added in the economy. This represents the National Income by the value-added method.

Advantages:
- The value-added method helps in avoiding double-counting by focusing on the value added at each stage of production.
- It provides a comprehensive and detailed analysis of the different sectors and their contribution to the economy.
- The method is suitable for economies with complex production processes and multiple stages of production.

Conclusion:
The value-added method, also known as the product method, is an important approach to calculating National Income. It focuses on the value added at each stage of production and helps in avoiding double-counting. By summing up the value added in the economy, the method provides an accurate measure of the total output and income of a country.