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The Gompertz distribution is a flexible distribution that can be skewed to the right and to the left. Its hazard function = is a convex function of (;,).The model can be fitted into the innovation-imitation paradigm with = as the coefficient of innovation and as the coefficient of imitation.
The hazard rate at time is the probability per short time dt that an event will occur between and + given that up to time no event has occurred yet. For example, taking a drug may halve one's hazard rate for a stroke occurring, or, changing the material from which a manufactured component is constructed, may double its hazard rate for failure.
In full generality, the accelerated failure time model can be specified as [2] (|) = ()where denotes the joint effect of covariates, typically = ([+ +]). (Specifying the regression coefficients with a negative sign implies that high values of the covariates increase the survival time, but this is merely a sign convention; without a negative sign, they increase the hazard.)
The hazard function is defined as the event rate at time , conditional on survival at time . Synonyms for hazard function in different fields include hazard rate, force of mortality ( demography and actuarial science , denoted by μ {\displaystyle \mu } ), force of failure, or failure rate ( engineering , denoted λ {\displaystyle \lambda } ).
The Gompertz curve or Gompertz function is a type of mathematical model for a time series, named after Benjamin Gompertz (1779–1865). It is a sigmoid function which describes growth as being slowest at the start and end of a given time period.
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This maximum likelihood maximization depends on the specification of the baseline hazard functions. These specifications include fully parametric models, piece-wise-constant proportional hazard models, or partial likelihood approaches that estimate the baseline hazard as a nuisance function. [4]
The 'bathtub curve' hazard function (blue, upper solid line) is a combination of a decreasing hazard of early failure (red dotted line) and an increasing hazard of wear-out failure (yellow dotted line), plus some constant hazard of random failure (green, lower solid line). The bathtub curve is a particular shape of a failure rate graph.