La Tasa Libre Riesgo - III

Como se verá más adelante, este autor está partiendo desde un supuesto particular: que el inversionista mantiene su inversión a lo largo de la vida del proyecto. Este punto de vista, si bien es respetable, no es compartido por todos los autores, implícita o explícitamente. Por ejemplo, Ehrhardt [1994] sostiene que la tasa libre de riesgo debe ser calculada considerando que el CAPM es un modelo de un solo periodo, y que por lo tanto el problema es determinar cual es la duración de este periodo. Aunque no existe una respuesta definitiva sobre el periodo aplicable al CAPM, se considera razonable asumir que este periodo es de corto plazo y por tanto se debe utilizar una tasa libre de riesgo de corto plazo [Ehrhardt 1994:60]

“There are many different securities that could be candidates for a proxy of the risk-free rate. What characteristics should a "good" candidate have? First, it should have no default risk. For all practical purposes, this limits your choice to U.S. Treasury securities. Which Treasury security should you choose? To answer this question, you need to think about the CAPM. There are many assumptions underlying it, and one of these assumptions states that CAPM is a one-period model. When you choose a risk-free rate, it seems reasonable that the period over which the risk -free rate is measured ought to correspond to the length of the CAPM period. But what is the appropriate CAPM period? Is it a day, a week, a month, a quarter, a year, or some longer period? Unfortunately, the re is no definitive answer to this question. If you use daily or monthly data to estimate CAPM, which is reasonable, you are implicitly assuming that the CAPM period is fairly short. Therefore, it is reasonable to use a shortterm risk-free rate.” [Ehrhardt 1994:60]