Now, to have a comprehensive idea of the total flow of production in the economy, we need to have a quantitative measure of the aggregate level of final goods produced in the economy. However, in order to get a quantitative assessment – a measure of the total final goods and services produced in the economy – it is obvious that we need a common measuring rod. We cannot add meters of cloth produced to tones of rice or number of automobiles or machines. Our common measuring rod is money. Since each of these commodities is produced for sale, the sum total of the monetary value of these diverse commodities gives us a measure of final output. But why are we to measure final goods only? Surely intermediate goods are crucial inputs to any production process and a significant part of our manpower and capital stock are engaged in production of these goods. However, since we are dealing with value of output, we should realize that the value of the final goods already includes the value of the intermediate goods that have entered into their production as inputs. Counting them separately will lead to the error of double counting. Whereas considering intermediate goods may give a fuller description of total economic activity, counting them will highly exaggerate the final value of our economic activity. At this stage it is important to introduce the concepts of stokes and flows. Often we hear statement like the average salary of someone is Rs 10,000 or the output of the steel industry is so many tones or so many rupees in value. But these are incomplete statements because it is not clear whether the income which is being referred to be yearly or monthly or daily income and surely that makes a huge difference. Sometimes, when the context is familiar, we assume that the time period is known and therefore do not mention it. But inherent in all such statements is a definite period of time. Otherwise such statements are meaningless. Thus income, or output, or profits are concepts.