R: Error when using Aggregate function to compile monthly means into yearly means
You can use the group_by() and summarize function from the dplyr package to achieve the above easily:

library(dplyr)
import.list &lt;- ldply(filenames, read_csv_filename)

# No need to create b1, b2 and b3
#by1 &lt;- import.list$Source
#by2 &lt;- import.list$Result
#by3 &lt;- import.list$Year

# here you want to group by the data based on 'Year' as you mentioned you want the yearly mean and you want to calculate mean of 'Result' right?.

import.list %&gt;%
  group_by(Year) %&gt;%
  summarize(Mean = mean(Result, na.rm=TRUE))-&gt;Final_Output



The above will group_by the data based on Year and calculate mean of Result column.