Why does `summarize` drop a group?

Question

I\'m fooling around with babynames pkg. A group_by command works, but after the summarize, one of the groups is dropped from the group

Answer 1

The behavior is noted in the documentation see ?summarise Value section:

Value An object of the same class as .data. One grouping level will be dropped.

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In contrast, mutate does not drop any grouping levels:

Value: An object of the same class as .data.