In Accounts
In accounting, the change in stock is calculated by subtracting the closing stock value from the opening stock value. This is because the focus is on determining the net change in stock during a specific period.

In Economics
In economics, when calculating national income, the approach is different. Here, the change in stock is calculated by subtracting the opening stock value from the closing stock value. This is because the focus is on determining the value of goods and services produced during a specific period, rather than the net change in stock.

Explanation
The difference in approach between accounts and economics lies in the perspective from which the calculation is made. Let's delve into the reasons behind this difference:

Accounts Perspective
1. Focus on stock valuation: In accounting, the primary purpose is to maintain accurate records of the financial position of a business. Therefore, the focus is on determining the value of stock at the beginning and end of a period, in order to calculate the change in stock.

2. Reflecting stock movements: By subtracting the closing stock value from the opening stock value, the change in stock can be calculated, which provides insights into the movement of stock during a particular period. This information is useful for inventory management, cost control, and financial reporting purposes.

Economics Perspective
1. Focus on production and income: In economics, the focus shifts from stock valuation to the measurement of national income. The goal is to determine the value of goods and services produced within an economy during a specific period.

2. Accounting for value addition: By subtracting the opening stock value from the closing stock value, economists can capture the value added to the stock during the period. This value addition is a result of the production process and represents the contribution of various factors of production, such as labor and capital.

3. Avoiding double counting: If the change in stock was calculated by subtracting the closing stock value from the opening stock value (as done in accounts), it could lead to double counting. This is because the production of goods and services already includes the value of intermediate goods used, and including the change in stock as well would result in counting the value of these goods twice.

Conclusion
In conclusion, the difference in calculating the change in stock between accounts and economics arises from their distinct objectives. Accounts focus on stock valuation and monitoring, while economics focuses on measuring national income and avoiding double counting. Understanding these differences is essential for applying the appropriate calculation method in each field.