This paper as a synthetic analyzis based on the Markowitz and Sharpe models deals with the problem of portfolio trying to determine both the optimum proportion of titles and the influence of a considered macroeconomic factor over their level of efficiency and risk. The improvement of these models was made through the introduction of a new model. the APT model (Arbitrage Price Theory).... https://countryscenesaddleryandpetsuppliers.shop/product-category/western-pads-tough-1/