Meaning of "Overdispersion" in Statistics I am trying to understand what "overdispersion" means in statistics.
Based on the Wikipedia page, "overdispersion" is defined as follows : "In statistics, overdispersion is the presence of greater variability (statistical dispersion) in a data set than would be expected based on a given statistical model."
However, I have heard other interpretations of "overdispersion" which suggest that "overdispersion refers to situations where the variance within the data is a function of the mean" - in other words, there is a non-constant relationship between the mean and variance within the data.
My Question: Can someone please tell how to mathematically measure and define "overdispersion"? For instance, I have heard that the Normal Distribution and the Poisson Distribution can be defined as "Dispersion Models". I have also heard that many models can be considered as "Dispersion Models" so long as a "Dispersion Parameter" can be inserted into the model. Using these definitions - is the Normal Distribution an example of "Overdispersion"? For example, there is a lot more variation in a Normal Distribution around the peak - and relatively less variation in a Normal Distribution around the tails. Is all this correct?
References:

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*https://en.wikipedia.org/wiki/Overdispersion

Over-dispersion can occur with one-parameter distributions, where mean and variance are tied together (Poisson, Binomial, Exponential). In real data, variance is usually much greater than would be allowed. Over-dispersion creates over-confidence (e.g. too narrow CIs), but usually does not introduce biases. In practical modelling, this problem  can be resolved in one of three ways:

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*quasi-likelihood or generalized equation estimation

*two-parameter distributions, such as negative-binomial or beta-binomial

*observation-level random effects

I am discussing the issue and solutions 2 & 3 in my book.
