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fix typo (#194) 

single typo edit
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nwaff 2016-07-26 05:42:04 -07:00 committed by Hadley Wickham
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transform.Rmd
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@ -483,7 +483,7 @@ summarise(flights, delay = mean(dep_delay, na.rm = TRUE))
(we'll come back to what that `na.rm = TRUE` means very shortly.)
`summarise()` is terribly useful unless we pair it with `group_by()`. This changes the unit of analysis from the complete dataset to individual groups. Then, when you use the dplyr verbs on a grouped data frame they'll be automatically applied "by group". For example, if we applied exactly the same code to a data frame grouped by date, we get the average delay per date:
`summarise()` is not terribly useful unless we pair it with `group_by()`. This changes the unit of analysis from the complete dataset to individual groups. Then, when you use the dplyr verbs on a grouped data frame they'll be automatically applied "by group". For example, if we applied exactly the same code to a data frame grouped by date, we get the average delay per date:
```{r}
by_day <- group_by(flights, year, month, day)