Discrete Models

Discrete Models

Suppose Note_1 that a time series of $q+1$ data points
MATH
is given. A likelihood function $L$ gives the probability that the observed data would result from the proposed stochastic mechanism relative to all other possible outcomes [132]. The data $y_{t}$ is a realization of the random variable $x\left( t\right) $. On the log scale, $w_{t}=\ln y_{t}$ is a realization of the random variable MATH The likelihood function $L$ is
MATH
where MATH is the joint probability distribution function (pdf) that $w_{t}$ occurs given that $w_{t-1}$ oc-curs. This is a normal pdf with mean MATH and variance $v$. Thus,
MATH
and
MATH
The maximum likelihood parameter estimates are those values of the parameters MATH that maximize MATH, or equivalently that maximize MATH. A calculation shows
MATH
where
MATH
are the log-residuals. The critical points MATH of $l$ are zeroes of the derivatives
MATH
i.e., are roots of the uncoupled equations
MATH

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