The job of econometrics is to specify and quantify these relationships. That is, econometricians formulate a statistical model, usually based on economic theory, confront it with the data, and try to come up with a specification that meets the required goals. The unknown elements in the specification, the parameters, are estimated from a sample of available data. Another job of the econometrician is to judge whether the resulting model is 'appropriate'. That is, check whether the assumptions made to motivate the estimators (and their properties) are correct, and check whether the model can be used for what it is made for. For example, can it be used for prediction or analysing policy changes? Often, economic theory implies that certain restrictions apply to the model that is estimated. For example, (one version of) the efficient market hypothesis implies that stock market returns are not predictable from their own past. An important goal of econometrics is to formulate such hypotheses in terms of the parameters in the model and to test their validity.