Stocks and flows

Total public debt is the accumulated outstanding liability measured at a particular date, so it is a stock. Fiscal deficit is the excess of government expenditure over receipts during a financial year, so it is a flow. A deficit is period-based even though it changes the debt stock.

Net investment is gross investment minus depreciation. When net investment is positive, additions to capital exceed the loss of capital through depreciation, so the capital stock increases. The capital stock remains a stock; investment is the flow that changes its level over time.

Raw-material stock at the end of the year is the quantity held at a specified date, making it a stock variable. Raw material purchased during the year is an amount acquired over a period, making it a flow. Purchases may alter inventory, but their classification remains flow-based.

Gross capital formation records investment and additions to productive assets over the course of a year, so it is measured as a flow. Capital stock at year-end records the amount of capital existing on a particular date, so it is a stock. The timing words determine the classification.

Total assets measured on 1 April represent the amount owned by the company at a specific point in time, so they form a stock. Profit earned over the year is measured across a period, so it is a flow. The date and duration indicators make option A unambiguous.

A machine is a capital stock that helps generate output over a period, so it has a stock-to-flow relationship. Investment is a flow during a period that adds to or expands the capital stock, creating a flow-to-stock relationship. Thus option A correctly shows both directions.

A fiscal deficit is a flow because it is measured for a particular financial year. If deficits continue, the government generally borrows to finance them, and successive borrowing accumulates into the stock of public debt. Thus a continuing deficit can increase public debt, although the exact change also depends on repayments and other adjustments.

National wealth at year-end is a stock measured at a particular date, whereas annual GDP is a flow generated during a year. Adding them directly combines quantities with different time dimensions and units of interpretation, creating a conceptual accounting error. The other options use appropriate time references.

Income is ordinarily a flow because it is earned over a period, such as a month or a year. A date in the statement may be only the reporting or closing date and does not automatically convert income into a stock. The context must be checked to determine whether the figure is period income or an actual balance available on that date.

The safest method is to examine both the time reference and what the quantity represents. A stock records a position at a point in time, while a flow records an activity, amount, or change over a period. The same economic area may contain both types; for example, capital is a stock while investment during a year is a flow. Therefore B is correct.

Capital stock consists of productive assets such as machines, buildings, and equipment. A larger or better-quality capital stock can increase productive capacity and therefore may permit a larger flow of output over a period. The word ‘may’ is important because technology, labour, and demand also affect output.