# Creating a Variable with Group Calculations

Many data sets have hierarchical structures, where individual observations belong within certain groups. For example, data with students inside classrooms inside schools. Or companies inside countries. See the below table for one example

I | J | X |
---|---|---|

1 | 1 | 3 |

1 | 2 | 3.5 |

2 | 1 | 2 |

2 | 2 | 5 |

Here, we have data where each group \(I\) has multiple rows, one for each \(J\).

We often might want to create a new variable that performs a calculation *within* each group, and assigns the result to each value in that group. For example, perhaps we want to calculate the mean of \(X\) within each group \(I\), so we can know how far above or below the group average each observation is. Our goal is:

I | J | X | AvgX |
---|---|---|---|

1 | 1 | 3 | 3.25 |

1 | 2 | 3.5 | 3.25 |

2 | 1 | 2 | 3.5 |

2 | 2 | 5 | 3.5 |

## Also Consider

- If the goal is to produce a data set with
*one row per group*rather than*a new variable assigning the calculation to each observation in the group*, then instead look at Collapse a Data Set.

# Implementations

## Julia

The **DataFrames.jl** package makes data aggregation and manipulaion relatively straigthforward.

```
# Load required packages
using CSV # Import .csv files
using DataFrames # Working with data frames
using Statistics # Required to calculate a mean
# Import .csv file from GitHub and store as a DataFrame
storms = CSV.read(download("https://vincentarelbundock.github.io/Rdatasets/csv/dplyr/storms.csv"), DataFrame)
# Use 'groupby' to aggregate data by groups (namely: name, year, month and day
# columns) and use 'transform!' to add a new column called 'mean_wind'
# containing the mean of the existing 'wind' column. (The `!` means that this
# change will be made in-place.)
transform!(groupby(storms, [:name, :year, :month, :day]), :wind=> mean => :mean_wind)
```

## Python

**pandas** doesn’t have a straightforward and flexible built-in method for doing this, with aggregation methods heavily preferring to work as described on Collapse a Data Set. However, we can just follow those methods and then `merge`

the result back in.

```
import pandas as pd
# Pull in data on storms
storms = pd.read_csv('https://vincentarelbundock.github.io/Rdatasets/csv/dplyr/storms.csv')
# Use groupby and agg to perform a group calculation
# Here it's a mean, but it could be anything
meanwind = (storms.groupby(['name', 'year', 'month', 'day'])
.agg({'wind': 'mean'})
# Rename so that when we merge it in it has a
# different name
.rename({'wind': 'mean_wind'}))
# make sure it's a data frame so we can join it
# Use merge to bring the result back into the data
storms = pd.merge(storms,meanwind,
on = ['name', 'year', 'month', 'day'])
```

## R

In R, we can use either the **dplyr** or **data.table** package to do this.

Here’s how to do it with **dplyr**…

```
library(dplyr)
data("storms") # The dataset is bundled with dplyr, so we'll just open directly
# Use 'group_by' to designate the groups and 'mutate' to create new column(s).
# Note that dplyr doesn't modify in-place, so we need to reassign the result.
storms = storms %>%
group_by(name, year, month, day) %>%
mutate(mean_wind = mean(wind))
```

…and here’s how to do it with **data.table**.

```
library(data.table)
# storms = fread("https://vincentarelbundock.github.io/Rdatasets/csv/dplyr/storms.csv")
setDT(storms) # Set the already-loaded storms DF as a data.table
# Use ':=' for in-place modification
storms[, mean_wind := mean(wind), by = .(name, year, month, day)]
```

## Stata

This process is made easy in Stata using the `egen`

command. `egen`

is not entirely flexible and is limited to a set of predetermined group calculations (see `help egen`

), although this can be expanded using the **egenmore** package (`ssc install egenmore`

then `help egenmore`

). However, the list of predetermined functions is long enough that you’ll rarely be caught out!

```
import delimited "https://vincentarelbundock.github.io/Rdatasets/csv/dplyr/storms.csv", clear
* Use bysort to determine the grouping, and egen to do the calculation
bysort name year month day: egen mean_wind = mean(wind)
```