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First, How can we derive the variance function with respect to mean for a given data?

Secondly, I would like to ask: what method should we employ if the variance in time series behaves like a high order (such as ##ππ’_π‘^5+ππ’_π‘^4+ππ’_π‘^3## polynomial function with respect to mean? On internet, I have always encountered the case where variance is a function of ##π’_π‘^2 or π’_π‘^4## like in this link I have not seen a case that variance is a function such as ##ππ’_π‘^5+ππ’_π‘^4+ππ’_π‘^3## . What should we do for the last case? How do we find the optimal power transformation or optimal other transformation methods to reduce heteroscedasticity?