The error terms are in the probabilistic sense.

In probabilistic sense random error are independent. Also, assumptions for random error, epsilon are mean equal to 0, variance equal to sigma squared and normal distribution. General form of probabilistic is y=deterministic+random error where y is the variable of interest. We always assume that the mean value of the random error equals 0. E(y) =mean of y, E(y) =deterministic component. 

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