The R Book

Summarizing the Contents of Dataframes

There are three useful functions here

• summary	summarize all the contents of all the variables
• aggregate	create a table after the fashion of tapply
• by	perform functions for each level of specified factors

Use of summary and by with the worms database on p. 110.

The other useful function for summarizing a dataframe is aggregate. It is used like tapply (see p. 18) to apply a function (mean in this case) to the levels of a specified categorical variable (Vegetation in this case) for a specified range of variables (Area, Slope, Soil.pH and Worm.density are defined using their subscripts as a column index in worms[,c(2,3,5,7)]):

aggregate(worms[,c(2,3,5,7)],by=list(veg=Vegetation),mean)

          veg      Area     Slope   Soil.pH  Worm.density
1      Arable  3.866667  1.333333  4.833333      5.333333
2   Grassland  2.911111  3.666667  4.100000      2.444444
3      Meadow  3.466667  1.666667  4.933333      6.333333
4      rchard  1.900000  0.000000  5.700000      9.000000
5       Scrub  2.425000  7.000000  4.800000      5.250000

The by argument needs to be a list even if, as here, we have only one classifying factor. Here are the aggregated summaries for Vegetation and Damp:

aggregate(worms[,c(2,3,5,7)],by=list(veg=Vegetation,d=Damp),mean) veg d Area Slope Soil.pH Worm.density 1 Arable FALSE 3.866667 1.333333 4.833333 5.333333 2 Grassland FALSE 3.087500 3.625000 3.987500 1.875000 3 Orchard FALSE 1.900000 0.000000 5.700000 9.000000 4 Scrub FALSE 3.350000 5.000000 4.700000 7.000000 5 Grassland TRUE 1.500000 4.000000 ...