Stocks and flows

National income is the total income earned by the normal residents of a country during a specified period, usually one financial year. Therefore, it is a flow variable, not a stock measured on one date. Bank balance and capital stock are stocks because they are recorded at a particular point of time.

A flow is measured over a period such as a month or year. Depreciation is the reduction in the value of machines, buildings, and other capital assets because of wear and tear or obsolescence during a period. Hence depreciation is a flow, whereas the value of assets on a date is a stock.

A stock refers to an economic quantity measured at a particular point of time, such as wealth, capital, inventory, or government debt on 31 March. A flow refers to a quantity measured during a specified period, such as annual income, savings, production, or tax collection. Therefore, option B correctly distinguishes the two concepts. The words “at a point” indicate a stock, while “during a period” indicates a flow.

A decrease in inventory during a year is measured over the whole year, not at one particular instant. It therefore represents a flow variable. The inventory existing on a specific date, such as the closing inventory on 31 March, is a stock. The size of the decrease may be negative, but its measurement over a period still makes it a flow.

Saving during a year is measured over a period, so it is a flow. Accumulated savings represent the amount built up and held at a particular point in time, so they are a stock. Option B reverses the order, while option C also places capital first as a stock. In option D, the order is stock followed by flow.

Stocks and flows are related: flows represent additions to or withdrawals from a stock over time. For example, deposits and interest can increase a bank balance, while withdrawals reduce it. Thus a flow can change a stock, although not every stock changes with every possible flow.

Investment measures the addition to the capital stock during a specified period, usually a year. Because it records an increase occurring over time, investment is a flow variable. Capital stock itself is the total value of productive assets existing at a point in time, while depreciation is a flow reducing that stock.

Foreign exchange reserves are an accumulated amount held at a particular point in time, so they are a stock. Exports are recorded over a period, such as a month, quarter, or year, so they are a flow. Option D correctly classifies both examples. The other options reverse the categories or incorrectly label balances and population-related quantities as flows.

Wealth, capital, and bank balance are all amounts measured at a particular point in time, so they are stocks. Annual income, monthly consumption, exports, investment, saving, production, wages, rent, and sales are measured during stated periods and are therefore flows. Option C is the only choice containing three stock variables without a period-based measurement.

The value of machines held at the end of the year is measured at a particular date, namely the year-end. It is therefore a stock of capital assets. The purchase of machines during the year would be investment and a flow, but the total machines owned at the date are a stock.

GDP measures the market value of final goods and services produced within an economy during a specified period, commonly one year. Since production is recorded across that interval rather than at one instant, GDP is a flow variable. A date-specific asset value would instead be a stock.

The total value of a country’s capital assets on a particular date represents the capital stock. It includes the existing stock of machines, buildings, equipment, and other productive assets at that moment. Capital formation or investment during a year would be a flow, not the asset total on a date.

The net increase in capital during a year is capital formation, and because it is measured over the year, it is a flow. It represents additions to the capital stock after considering relevant reductions such as depreciation. The capital stock itself is the total capital existing at a particular point in time.

Depreciation is the loss or reduction in the value of capital during a period, so it is a flow. Capital stock is the total value of capital existing at a particular point in time, so it is a stock. The key is to distinguish a change occurring during a period from the accumulated amount at a date.

Capital stock at the beginning of the year is measured at a specific point, namely the start of the year, so it is a stock. Investment during the year is expenditure occurring over a period, so it is a flow. Investment may increase the capital stock, but the two variables have different time references.

Tax collection is measured as the amount collected during a period, such as a month or financial year, so it is a flow. Outstanding tax liability is the total unpaid amount existing at a particular date, so it is a stock. The word outstanding signals an accumulated balance rather than a period transaction.

Government debt is a stock because it is the total outstanding liability accumulated up to a particular date. The annual fiscal deficit is a flow because it measures the excess of government expenditure over receipts during one financial year and indicates borrowing required in that period. Thus, debt is a stock and the annual deficit is a flow, making option B correct.

Exports recorded during a year are a flow because they represent an economic activity measured over a period of twelve months. Foreign exchange reserves reported at the end of the year are a stock because they indicate the accumulated amount held at a particular point in time. Therefore, exports are a flow and reserves are a stock, so option B is correct.

Income, consumption expenditure, and investment are flow variables because each is measured over a period such as a month, quarter, or year. The other options contain stock variables: wealth, population, bank balance, capital stock, inventory, money supply, and debt are measured at a point in time. Therefore, only option A contains flow variables.

A machine is a capital asset available at a point of time, so it is a stock. The output or productive services generated by that machine during a year are measured over a period, so they are flows. This illustrates that a stock of assets can produce a stream of services or output.

Investment is expenditure on adding to or replacing productive assets during a period, so it is a flow. When investment adds new machines, buildings, or equipment, it increases the capital stock available at a later point of time. Depreciation can reduce that stock, showing the opposite flow effect.

The opening cash balance is measured at a particular point in time, namely the beginning of the year, so it is a stock variable. Cash receipts are recorded over the duration of the year, so they are a flow variable. A stock shows a quantity at a date, while a flow shows activity during a period.

The value of machines on 31 March is measured at a specific point of time, so it is a stock. A stock represents the quantity of an asset existing at a particular date. In contrast, the value of machines purchased during the year is measured over a period of time and therefore represents a flow. Thus, the correct sequence is stock and flow, given in option A.

A household’s bank balance today is measured on a particular date, so it is a stock. It indicates the amount held at that point of time. The total amount deposited during a month is measured over an interval, not at one instant, so it is a flow. Therefore, the correct classification is stock and flow, represented by option B.

The population of a city on a particular day is measured at a definite point of time. It is therefore a stock, even though population changes continuously. Births occurring during a year are counted over a period of time and represent an addition during that interval. They are therefore a flow. Hence, option D is correct.