[WIP] Clarify errors, residuals, deviations, and differences

[Copilot]

Thanks for asking me to work on this. I will get started on it and keep this PR's description up to date as I form a plan and make progress.

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This section details on the original issue you should resolve

<issue_title>define and clarify the relationships among errors, residuals, deviations, and differences</issue_title>
<issue_description>"deviation" is a difference between a given value and a reference value; typically, it is used as a shorthand for "deviation of a random variable from its population mean (i.e. expectation)". "deviation" can apply to both data and estimators.

"error" is typically used to refer to deviations of observations from their population means.

"residual" is typically used to refer to deviations between observations and their fitted values (which is often but not always the sample mean)

refer readers to
https://en.wikipedia.org/wiki/Errors_and_residuals
and
https://en.wikipedia.org/wiki/Deviation_(statistics)
for more detail

put deviation and error in the probability chapter and residual in the estimation chapter, and put the comparison of residual vs error in the estimation chapter (after the definition of residual). Some of these definitions may already exist; don't duplicate them, just move them around and amend them as needed. check those wikipedia articles to make sure we get the definitions and relationships correct.</issue_description>

Comments on the Issue (you are @copilot in this section)

[d-morrison]

wrong repo