In statistics, the likelihood function (often simply the likelihood) is the joint probability distribution of observed data expressed as a function of statistical parameters.
It describes the relative probability or odds of obtaining the observed data for all permissible values of the parameters, and is used to identify the particular parameter values that are most plausible given the observed data.
en.wikipedia.org/wiki/Likelihood_function
Many probability distributions have unknown parameters.
We estimate these unknowns using sample data.
The Likelihood function gives us an idea of how well the data summarizes these parameters.
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