Supporting Statistical Analysis for Research

## 6.3 Aggregating data

### 6.3.1 Data concepts

Data aggregation is reducing a set of values to a smaller set of values (typically a single value) value through some function of the individual values. A common statistical data aggregation is reducing a distribution of values to a mean and standard deviation. Another example of data reduction is frequency tables.

Aggregating data is a useful tool for data exploration. A histogram is an example of aggregation for exploration. Histograms count (aggregate) the number of observations that fall into bins. While some data is lost in this aggregation, it also provides a very useful visualization of the distribution of a set of values.

Aggregation is sometimes done to allow for analysis to be completed at a higher level of the data. For example, if an analysis of the size of school districts in a region is to be done, the number of students from the schools with in the district are summed (aggregated.)

### 6.3.2 Examples - R

These examples use the Blackmore.csv data set.

1. We begin by loading the tidyverse and import the csv file.

library(tidyverse)
blackmore_path <- file.path("..", "datasets", "Blackmore.csv")
blackmore_in <- read_csv(blackmore_path, col_types = cols())
Warning: Missing column names filled in: 'X1' [1]
blackmore <-
blackmore_in %>%
select(-X1)

glimpse(blackmore)
Observations: 945
Variables: 4
$subject <chr> "100", "100", "100", "100", "100", "101", "101", "101...$ age      <dbl> 8.00, 10.00, 12.00, 14.00, 15.92, 8.00, 10.00, 12.00,...
$exercise <dbl> 2.71, 1.94, 2.36, 1.54, 8.63, 0.14, 0.14, 0.00, 0.00,...$ group    <chr> "patient", "patient", "patient", "patient", "patient"...
2. Some of the ages have fractional components. We will round down, the floor function, the age variable to find each subjects integer age.

blackmore <-
blackmore %>%
mutate(
age = floor(age)
)

glimpse(blackmore)
Observations: 945
Variables: 4
$subject <chr> "100", "100", "100", "100", "100", "101", "101", "101...$ age      <dbl> 8, 10, 12, 14, 15, 8, 10, 12, 14, 16, 8, 10, 12, 15, ...
$exercise <dbl> 2.71, 1.94, 2.36, 1.54, 8.63, 0.14, 0.14, 0.00, 0.00,...$ group    <chr> "patient", "patient", "patient", "patient", "patient"...
3. Find the mean exercise by group, the treatment variable.

mean_exercise <-
blackmore %>%
group_by(
group
) %>%
summarize(mean = mean(exercise))

mean_exercise %>%
as.data.frame()
group     mean
1 control 1.640641
2 patient 3.075887

The use of as.data.frame() was used here to provide a more table like display.

4. The mean within the treatment groups might vary over subject age. We will find the mean exercise by treatment and the last year a subject was in the program.

last_age <-
blackmore %>%
group_by(
subject
) %>%
filter(
age == max(age)
) %>%
ungroup()
mean_last_age_exercise <-
last_age %>%
group_by(group, age) %>%
summarize(mean = mean(exercise))

mean_last_age_exercise %>%
as.data.frame()
group age     mean
1  control  11 1.520000
2  control  12 1.730714
3  control  13 1.424545
4  control  14 2.381875
5  control  15 2.240588
6  control  16 2.505000
7  control  17 2.124444
8  patient  11 1.840000
9  patient  12 6.096000
10 patient  13 5.124211
11 patient  14 5.185667
12 patient  15 8.282857
13 patient  16 5.473488
14 patient  17 6.395000

The way the data is displayed makes it difficult to compare the two groups. We will put the two groups into side by side columns in a table.

mean_last_age_exercise <-
last_age %>%
group_by(group, age) %>%
summarize(mean = mean(exercise)) %>%
spread(key = group, value = mean)

mean_last_age_exercise %>%
as.data.frame()
age  control  patient
1  11 1.520000 1.840000
2  12 1.730714 6.096000
3  13 1.424545 5.124211
4  14 2.381875 5.185667
5  15 2.240588 8.282857
6  16 2.505000 5.473488
7  17 2.124444 6.395000
5. We will repeat the prior problem and including the number of subjects in each group.

mean_last_age_exercise <-
last_age %>%
group_by(
group,
age
) %>%
summarise(
mean = mean(exercise),
number = n()
) %>%
gather(key = stat, value = value, mean, number) %>%
unite(stat_name, sep = "-", group, stat) %>%
spread(key = stat_name, value = value)

mean_last_age_exercise %>%
as.data.frame()
age control-mean control-number patient-mean patient-number
1  11     1.520000              1     1.840000              1
2  12     1.730714             14     6.096000              5
3  13     1.424545             22     5.124211             19
4  14     2.381875             16     5.185667             30
5  15     2.240588             17     8.282857             28
6  16     2.505000             14     5.473488             43
7  17     2.124444              9     6.395000             12
6. Make a histogram of the distribution of the size of the last age when grouped by age and treatment.

blackmore_group_size <-
last_age %>%
group_by(
group,
age
) %>%
summarise(
count = n()
) %>%
ungroup()

ggplot(blackmore_group_size, aes(x = count)) +
geom_histogram(binwidth = 1) +
theme_bw()

### 6.3.3 Examples - Python

These examples use the Blackmore.csv data set.

1. We begin by loading the packages and importing the csv file.

import pandas as pd
import os
import numpy as np
blackmore_path = os.path.join('..', 'datasets', 'blackmore.csv')

blackmore =  (
blackmore_in
.copy(deep=True)
.drop(columns='Unnamed: 0'))

(blackmore
.pipe(print))
subject    age  exercise    group
0     100   8.00      2.71  patient
1     100  10.00      1.94  patient
2     100  12.00      2.36  patient
3     100  14.00      1.54  patient
4     100  15.92      8.63  patient
2. Some of the ages have fractional components. We will round down, the floor function, the age variable to find each subjects integer age.

blackmore = (
blackmore
.assign(age=lambda df: np.floor(df['age']).astype('int')))

(blackmore
.pipe(print))
subject  age  exercise    group
0     100    8      2.71  patient
1     100   10      1.94  patient
2     100   12      2.36  patient
3     100   14      1.54  patient
4     100   15      8.63  patient
3. Find the mean exercise by group, the treatment variable.

The mean() method for the GroupBy object will calculate the mean for each group. This is one of the aggregating methods for GroupBy.

mean_exercise = (
blackmore
.groupby('group')
['exercise']
.mean()
.reset_index())

(mean_exercise
.pipe(print))
group  exercise
0  control  1.640641
1  patient  3.075887
4. The mean within the treatment groups might vary over subject age. We will find the mean exercise by treatment and the last year a subject was in the program.

To find the last age for each participant, we sort by age and use the tail() method to retain only the largest age.

last_age = (
blackmore
.sort_values(['subject', 'age'])
.groupby('subject')
.tail(1)
.reset_index()
)
mean_last_age_exercise = (
last_age
.groupby(['group', 'age'])
['exercise']
.mean()
.reset_index()
)

(mean_last_age_exercise
.pipe(print))
group  age  exercise
0   control   11  1.520000
1   control   12  1.730714
2   control   13  1.424545
3   control   14  2.381875
4   control   15  2.240588
5   control   16  2.505000
6   control   17  2.124444
7   patient   11  1.840000
8   patient   12  6.096000
9   patient   13  5.124211
10  patient   14  5.185667
11  patient   15  8.282857
12  patient   16  5.473488
13  patient   17  6.395000

The way the data is displayed makes it difficult to compare the two groups. We will put the two groups into side by side columns in a table.

mean_last_age_exercise = (
last_age
.groupby(['group', 'age'])
['exercise']
.mean()
.reset_index()
.pivot(index='age', columns='group', values='exercise')
)

(mean_last_age_exercise
.pipe(print))
group   control   patient
age
11     1.520000  1.840000
12     1.730714  6.096000
13     1.424545  5.124211
14     2.381875  5.185667
15     2.240588  8.282857
16     2.505000  5.473488
17     2.124444  6.395000
5. We will repeat the prior problem and including the number of subjects in each group.

The built in aggregating functions for the GroupBy object are useful if you need a single aggregating function for each group. The aggregate() method is a more general purpose approach to aggregating data in groups. It is given a dictionary with an entry for each column that will have a aggregating function applied to it. A list of aggregating functions is given for each of the columns being aggregated.

The aggregate() method returns a column index in two rows (hierachtical index) when multiple functions are applied to a column.

mean_last_age_exercise = (
last_age
.groupby(['group', 'age'])
.aggregate(
{'exercise': ['mean', 'size']})
.reset_index()
.rename_axis(None, axis='index')
)
group age  exercise
mean size
0  control  11  1.520000    1
1  control  12  1.730714   14
2  control  13  1.424545   22
3  control  14  2.381875   16
4  control  15  2.240588   17
flat_col = [t[0] if t[1] == '' else t[1] for t in
mean_last_age_exercise.columns]
print(flat_col)
['group', 'age', 'mean', 'size']
mean_last_age_exercise.columns = flat_col
mean_last_age_exercise = (
mean_last_age_exercise
.pivot(
index='age',
columns='group',
values=['mean', 'size'])
.reset_index()
.rename_axis(None, axis='index')
)
age      mean              size
group       control   patient control patient
0      11  1.520000  1.840000     1.0     1.0
1      12  1.730714  6.096000    14.0     5.0
2      13  1.424545  5.124211    22.0    19.0
3      14  2.381875  5.185667    16.0    30.0
4      15  2.240588  8.282857    17.0    28.0
flat_col = [ t[0] if t[1] == '' else t[1] + '-' + t[0] for t in
mean_last_age_exercise.columns]
print(flat_col)
['age', 'control-mean', 'patient-mean', 'control-size', 'patient-size']
mean_last_age_exercise.columns = flat_col

(mean_last_age_exercise
.pipe(print))
age  control-mean  patient-mean  control-size  patient-size
0   11      1.520000      1.840000           1.0           1.0
1   12      1.730714      6.096000          14.0           5.0
2   13      1.424545      5.124211          22.0          19.0
3   14      2.381875      5.185667          16.0          30.0
4   15      2.240588      8.282857          17.0          28.0
5   16      2.505000      5.473488          14.0          43.0
6   17      2.124444      6.395000           9.0          12.0
6. Make a histogram of the distribution of the size of the last age when grouped by age and treatment.

last_age_group_size = (
last_age
.groupby(['group', 'age'])
['exercise']
.size()
.reset_index()
.rename(columns={'exercise': 'size'})
)
group  age  size
0  control   11     1
1  control   12    14
2  control   13    22
3  control   14    16
4  control   15    17
(p9.ggplot(last_age_group_size, p9.aes(x='size')) +
p9.geom_histogram(binwidth=1) +
p9.theme_bw())
<ggplot: (-9223371893255862198)>

### 6.3.4 Exercises

These examples use the Car.csv data set.

1. Load the Car.csv data set.

2. Create a categorical variable that identifies the body type that was selected first.

3. Create a table of the number of count of the selected types.

4. Create a table of the number of trucks selected by college degree or not and greater than 2 in the family.

Which of these subgroups selected truck as their first choice in greatest numbers.

5. Create the same table with the cells reporting the proportion of these groups that select trucks.

Which of these subgroups was most likely to selected truck as their first choice.

6. Create a variable that identifies the price of the first selected vehicle.

7. Create a table that provides the mean and standard deviation of price by college attendance or not groups.