an optional argument called na.rm, which is shorthand for “remove NA values”. By default, na.rm = FALSE, so R does nothing about the missing data problem. Let’s try setting na.rm = TRUE and see what happens: When calculating sums and means when missing data are present (i.e., when there are NA values) there’s actually an additional argument to the function that you should be aware of. This argument is called na.rm, and is a logical value indicating whether R should ignore (or “remove”) the missing data for the purposes of doing the calculations. By default, R assumes that you want to keep the missing values, so unless you say otherwise it will set na.rm = FALSE.

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## Error in Ops.factor(group, gender): level sets of factors are different

a paper by Ioannidis

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