(Requires some Calculus)Consider the sample regression function . 
.Take the total derivative.Next show that the partial derivative 
is obtained by holding 
constant,or controlling for 
.
Definitions:
Warehouse
A storage facility for housing goods in large quantities, typically before they are distributed for sale.
Return On Investment
A performance measure used to evaluate the efficiency or profitability of an investment, calculated as net profit divided by the cost of the investment.
Margin
The difference between the selling price of a product and the cost of the product, representing profitability.
Sales
The total revenue earned from the sale of goods or services related to a company's primary operations.
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