Stocks and flows

Foreign-exchange reserves reported on a particular date represent the stock of foreign assets available at that point in time. Export earnings during a year are receipts generated through exports over a period, so they are a flow. The date and the period wording establish the different classifications.

National wealth, money supply, and capital stock are all accumulated quantities measured at a point of time. Wealth is the value of assets, money supply is the amount of money outstanding, and capital stock is the available productive asset base. Therefore, all three in option A are stocks.

Income is earned during a period, investment is made during a period, and depreciation records the loss of capital value during a period. Therefore, all three are flows. The other options contain balances, wealth, population, or stocks measured at a point in time, so they are not sets of flow variables.

Capital stock consists of productive assets available at a particular point in time. Those assets provide productive services and can generate output during the year, and annual output is measured over a period, making it a flow. Thus option A correctly shows a stock followed by a related flow.

Total debt on a specified date is the accumulated outstanding liability of the economy at that point in time, so it is a stock. Borrowing during the year records new debt raised over a period, so it is a flow. Period borrowing may increase the outstanding debt stock, subject to repayments and other adjustments.

Fiscal deficit measures the government's excess expenditure over receipts during a specified financial year and therefore represents a borrowing requirement over a period. Public debt is the accumulated outstanding liability at a particular date, so it is a stock. Annual deficits can add to the public-debt stock.

Sales record the quantity or value of goods transferred to buyers during a period, so sales are a flow. Finished goods remaining with the firm are inventory available at a particular date, so they are a stock. Production and sales can change inventory, but the remaining inventory itself is a point-in-time balance.

Saving is the part of income not spent during a specified year, so it is measured over a period and is a flow. Financial assets held at the end of the year represent a balance existing at a particular date, so they are a stock. Saving may increase the stock of financial assets, although purchases, sales, and valuation changes can also affect it.

The number of unemployed persons on a specified date is a stock because it counts people in a condition at one point in time. The number of people obtaining jobs during a month records transitions occurring over a period, so it is a flow. The same people may affect both measures, but the time references differ.

Income is earned over a period, such as a month or year, so it is a flow. Money wealth is the accumulated value of money and assets owned at a particular time, so it is a stock. Income, after saving, can add to or reduce wealth, showing how a flow changes a stock.

Population existing on a particular date is a stock because it is measured at a point in time. The net increase in population during a year records a change occurring over a period; births minus deaths, with suitable migration adjustments, form a flow. Therefore, option C is correct.

Each capital-stock figure is measured at a particular point in time. Subtracting the beginning stock from the ending stock gives the change that occurred during the intervening period. A change over a period is a flow, although the difference may also reflect investment, depreciation, sales, or other adjustments.

The number of machines available today is measured on a particular date, so it is a stock of capital assets. Machines purchased during the month represent an acquisition occurring over a period, so they are a flow. The purchase may add to the stock, but the transaction itself remains a flow.

Net investment is measured during a year and represents the addition to capital after allowing for depreciation or other reductions. It is therefore a flow. Total machines, fixed assets today, and capital at year end are quantities existing at particular times and are therefore stocks.

Money supply refers to the quantity of money available at a particular time, so it is treated as a stock. Expenditure records spending carried out over a period, such as a month or year, and is therefore a flow. Their monetary unit does not determine their stock-flow classification.

Imports during a year record goods purchased from abroad over a period, so imports are a flow. Imported goods remaining at the end of the year are an inventory balance measured on a particular date, so that stock is a stock. A flow can add to an inventory stock, but the two concepts are not identical.

Annual national income measures income earned during the whole year, so it is a flow. The other options refer to wealth, bank balance or inventory existing at the end of the year. Those are measured at a particular point in time and are therefore stocks, despite the annual wording.

Gross investment measures expenditure on additions to capital goods during a period, so it is a flow. Capital stock is the total value or quantity of capital assets existing at a particular time, so it is a stock. Gross investment can increase the capital stock, while depreciation can reduce it.

The decisive criterion for distinguishing a stock from a flow is the time reference, not price, ownership, or origin. A stock is measured at one point, such as wealth on 31 March, whereas a flow is measured over a period, such as income during a year. Therefore, reading words like “at the end,” “on a date,” and “during” carefully is the essential precaution.

Total assets at the end of the year represent the value existing at a particular point in time, so they are a stock. Asset purchases are transactions occurring throughout the year and are measured over a period, so they are a flow. The classification depends on the time dimension, not on whether the assets are public or private.

National wealth means the total stock of assets owned by an economy at a particular date, so “today” gives it a point-in-time character. National income is earned through production and factor payments during a year, so it is a flow. Hence national wealth is a stock and national income is a flow.

Opening capital stock is the quantity of capital available at the beginning of a specified date, so it is a stock. Net investment records the addition to or reduction of capital over the course of the year, so it is a flow. The two terms differ because one is measured at a point and the other over a period.

External debt at the end of the year is the accumulated outstanding liability measured at a specific point in time, so it is a stock. In contrast, the current account deficit is calculated from transactions in goods, services, income, and transfers occurring throughout a year, so it is a flow. Therefore, the correct sequence is stock and flow. A stock is measured at a point, while a flow is measured over a period.

Opening inventory is the quantity of goods held by the firm at the beginning of a particular accounting period, so it is measured at a point in time and is a stock. The change in inventory records the increase or decrease between the beginning and end of the year; because it is measured over a period, it is a flow. Hence, the correct answer is stock and flow.

Saving is a flow because it is measured over a period, such as a month or year. When saving is retained rather than consumed, it adds to accumulated wealth, which is a stock measured at a point in time. Thus a flow, saving, can build up the stock of wealth. The other options do not show this standard accumulation relationship.