Expanded the group_by() and summarise() section with a few more advanced examples (which may or may not be covered, depending on time), namely:
mentioning that more than one summary can be done at once
grouping can be done on more than one variable
we can get the number of observations for each group with n() (I think this is important, because it's so useful)
if we just want the number of observations in a group we can use count()
Added a new (optional) section talking about spread() and gather() (a few people already seem to have manipulated their tables - in excel! - to a long format, so this might be useful for them to know)
I've reviewed the materials with the following:
group_by()andsummarise()section with a few more advanced examples (which may or may not be covered, depending on time), namely:n()(I think this is important, because it's so useful)count()spread()andgather()(a few people already seem to have manipulated their tables - in excel! - to a long format, so this might be useful for them to know)