You might think that we could use the newly defined function within the dplyr framework just like the other function inside the summarize() function, but there is a problem in using the multi-valued function within summarize(). Here is an example of the use of the customized multi-valued function within the summarize() function:

    desStat <- USAairlineData2016 %>%
    select(MONTH, ORIGIN, DEP_DELAY) %>%
    group_by(ORIGIN, MONTH) %>%
    summarise(  
    fourNumSum(DEP_DELAY)
    )

If you run the preceding code, you will get an error as shown in the following screenshot:

The summarise() verb is expecting a function that can produce a single-valued output, but the function that has been used is producing a multi-valued output, and as a result, the error is being generated by the R process.