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How could I decide from a variable exactly, that is stock or flow type?

Variables, which are measured at a point of time are called stock variables whereas variables measured over a period of time are flow variables. This means that stocks are non-recurring in nature whereas flows are recurring in nature.

I found, that number of unemployed people is stock and amount of salary is flow. As I know, salary is a fixed amount of money, but number of unemployed people can be changed really fast.

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A stock represents a quantity measured at one specific point in time, which may have accumulated in the past.

According to the Bureau of Labor Statistics, the US unemployment rate stood at 4.1 percent in February 2018. This represented a stock of 6,706,000 unemployed workers in February 2018. But from January to February, we observed a flow of workers going in and out unemployment.

A flow variable is thus measured over an interval of time, e.g., hours, days, weeks, months or years. It has time dimension.

Because of inflows and outflows of workers going in and out unemployment, between January and February, the stock of unemployed workers was a bit higher in January 2018 (=6,684,000).

A year before, in February 2017, the unemployment rate was about 4.7%, representing a stock of 7,486,000 unemployed workers at that particular time. This means that a net flow of 7,800 (=7,486,000 - 6,706,000) US workers went out unemployment between February 2017 and February 2018.

Your wage in February 2018 is a flow because it's the money you earn between Feb 1st and Feb 28. (by the way, this amount is not necessarily fixed from one month to another).

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