Identification is a problem of model formultion, rather than inf nlnde! estimation or appraisal. We say a model is identified if it is in a unique statistical form, enabling unique estimates of its parameters to be suhsequerltly made from sample data. If a model is not identified then we cannot say exactly what relationship we are estimating.
To measure the coefficients of the demand ecjuattlon: cornlally the published time series reporting the quantity bough of the corrtmodir). is used. tiowever, the quantity bought is identical with the quantity sold at any particular price. Market data register points of interaction of equilibrium supply and demand at the price prevailing in the market at a certairz point of time. A sample of time-series observations shows simultaneously the quantity demanded, and the quantity supplied, at the prevailing market price. That is. it only shows the points of interactions of demand and supply. If' we use these data for estimation, we actually measure the coefficients of a function of the form Q = f (p) . This equation may be either the demand function or the supply function. But how can we be sure whether this equation represents demand function or supply function? If anyone is interested to measure the demand function then he can use the data. Similarly, the person who is interested to measure the supply equation will also be using the same data. It is clear that we need some criteria, which will enable us to verify that the estimated coefficients belong to the one or the other relationship.