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In ordinary least squares regression, we assume that a linear change in an independent variable is related to a linear change in the dependent variable (note that the independent variable may be a transformed one). In proportional hazard models we assume that a linear increase in the independent variable is related to a multiplicative increase in the hazard. So, if the hazard when $x_1 = 2$ is double that when $x_1 = 1$ then we assume that the hazard when $x_1 = 3$ will be double that when it is 2.