where formula_8 is a set of regression coefficients and formula_9 is a set of independent variables (also known as "features") describing person "n", which may be either discrete "dummy variables" or regular continuous variables. formula_10 is a random variable specifying "noise" or "error" in the prediction, assumed to be distributed according to some distribution. Normally, if there is a mean or variance parameter in the distribution, it cannot be identified, so the parameters are set to convenient values — by convention usually mean 0, variance 1.