Stocks and flows

The expression “per month” refers to an amount, activity, income, expenditure, or change occurring over a time period. It therefore signals a flow variable. Words such as “today,” “at this time,” and “at year end” generally identify a position at a particular point in time, which is characteristic of a stock. Consequently, option B is the only option containing a clear period-based flow signal.

Monthly income is earned over the course of a month, so it is a flow. Calling it a stock is therefore a classification error. Year-end assets, today’s population, and closing inventory are all measured at particular points in time and are correctly treated as stocks. Hence, option A identifies the mistake.

Total debt means the accumulated amount outstanding at a particular date, such as the end of a financial year. Since it is measured at a point in time, it is a stock variable. Annual interest payment is the amount paid or incurred over the course of one year, so it is a flow variable. The interest flow may be calculated on the debt stock, but the two concepts remain different. Therefore, option A is correct.

Opening liabilities show the amount owed by the firm at the beginning of an accounting year, which is a position measured at one point in time. They are therefore a stock variable. Interest expense during the year accumulates over the accounting period and represents the cost incurred throughout that period, so it is a flow variable. The correct order is consequently stock and flow, as stated in option B. The time reference determines the classification.

The number of houses at the beginning of the year is a stock because it is measured at a particular point in time. New houses built during the year are a flow because construction is measured over a period of time. Thus, the correct order is stock and flow. In economics, quantities existing at a date are stocks, whereas additions, production, income, or expenditure occurring during a period are flows.

New houses built during the year are a flow because the number is measured over an interval of time, namely the year. Total houses at the end of the year are a stock because they represent the accumulated quantity existing at one specific point in time. Therefore, the correct order is flow and stock. The words ‘during the year’ usually indicate a flow, while ‘at year end’ indicates a stock.

Raw material remaining at the end of the year is an inventory balance measured on a particular date, so it is a stock. Raw material consumed during the year is measured across a time interval, so it is a flow. A stock records a position, whereas a flow records an activity or change occurring during a period.

The number of machines existing on a specified date is a capital position at one point in time, so it is a stock. The purchase of machines during the year is expenditure or acquisition occurring over a period, so it is a flow. Purchases may increase the stock, but the purchase activity itself remains a flow.

The amount of water present in the pond at a stated time is a stock because it records the level of a resource at one point. Water withdrawn each day is a flow because it measures movement or use over a daily period. The water analogy makes clear that flows can reduce or change a stock without becoming stocks themselves.

A natural-resource reserve is the quantity available or remaining at a particular point in time, so it is a stock. Annual extraction measures how much of the resource is removed during one year, so it is a flow. Extraction may reduce the reserve, but the activity of extraction remains a period-based flow.

A firm’s position, such as its assets or liabilities on a balance-sheet date, is measured at a point in time and is therefore a stock. A firm’s activity, such as sales, production, or expenditure during a month, is measured over a period and is therefore a flow. Hence option D gives the correct distinction.

Assets on 31 March are measured on one fixed date, so they represent a stock. Sales during March, income earned during a year, and wages earned during a week are all measured across periods and therefore represent flows. The words “on 31 March” provide the decisive point-of-time reference.

Output during a quarter is measured over a defined period, so it is a flow. Cash today, debt at the end of the year, and capital on a particular date are measured at specific points in time and are therefore stocks. The word “during” clearly signals an activity accumulated over an interval.

Capital stock at the beginning of the year is measured at the starting point in time, while capital stock at the end is measured at the ending point in time. Both are quantities of capital existing at particular dates, so both are stocks. The difference between them may be related to a flow such as net investment, but the two stated capital levels remain stocks.

Foodgrain stock at year end is measured at a particular point in time, so it is a stock. Foodgrain purchases are recorded over the course of the year, so they are a flow. A stock is a quantity existing at a date, whereas a flow is measured over a period. Therefore, the correct order is stock and flow.

The customer list gives the number of customers existing on a specified date, so it is a stock. New customers joining during a month are counted over a time interval, so they represent a flow. The classification depends on whether the information describes a position at one date or an addition during a period. Hence, option C is correct.

The expression “per week” specifies a rate or amount measured over a time interval. Quantities such as weekly income, weekly sales, or weekly production are therefore generally flows. A stock is normally stated as an amount existing at a particular date, without requiring a period such as a week. Thus, option B is the correct clue, although the actual definition should always be checked.

31 March identifies a specific point in time. A total amount available on that date describes a balance or position existing at that moment, so it is a stock. It is not measured over the whole year or month, as annual income and monthly expenditure would be. Therefore, the date-based amount is correctly classified as a stock, making option B correct.

A worker's bank balance today is the amount held at a particular point in time, so it is a stock. Wages earned during the week are measured over the weekly period, so they are a flow. The nature of the item is determined by its time reference, not by whether it belongs to a worker or is expressed in money. Hence, option B is correct.

Population counted on a census day is a total existing at a particular point in time, so it is a stock. Births during the year are events counted over an interval of time, so they are a flow. Births may change the population stock, but the annual number of births itself is a flow. Therefore, option A gives the correct classification.

Year-end inventory is the amount of goods remaining and available at a particular date, so it is a stock. Purchases made throughout the year are transactions occurring over a period, so they are a flow. The inventory balance may be affected by purchases and sales, but the ending balance and the annual purchases remain different measures. Therefore, option B is correct.

Monthly consumption is measured over the course of one month, so it is a flow. It records spending or use during a period rather than a balance existing on one date. Today's bank balance, year-end wealth, and population on a specified date are all point-in-time positions and therefore stocks. Thus, option A contains the incorrect identification.

Cash balance on a particular date is the amount available at that moment, so it is a stock. It is incorrectly called a flow in option B. Annual tax collection, yearly output, and monthly income are each measured over a period and are therefore flows. The key distinction is between a date-specific balance and an activity or receipt accumulated during a period.

A stock is measured at a particular point of time, whereas a flow is measured over a period. Beginning and ending capital stocks are two stock measurements. Their difference shows the net change in capital during the intervening period, so it is treated as a flow. For example, ending capital stock minus beginning capital stock indicates the period’s net addition or reduction.

A stock represents the quantity of an economic variable existing at a specific point in time. Therefore, total foreign capital present in a country on a specified date, such as 31 March, is a stock. Foreign investment received during a year or month is measured over a period and is a flow; income earned during a year is also a flow.