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    In [1]: import pandas as pd

Data used for this tutorial:

- Titanic data

  This tutorial uses the Titanic data set, stored as CSV. The data consists of the following data columns:

  - PassengerId: Id of every passenger.

  - Survived: Indication whether passenger survived. `0` for no and `1` for yes.

  - Pclass: One out of the 3 ticket classes: Class `1`, Class `2` and Class `3`.

  - Name: Name of passenger.

  - Sex: Gender of passenger.

  - Age: Age of passenger in years.

  - SibSp: Number of siblings or spouses aboard.

  - Parch: Number of parents or children aboard.

  - Ticket: Ticket number of passenger.

  - Fare: Indicating the fare.

  - Cabin: Cabin number of passenger.

  - Embarked: Port of embarkation.

  To raw data

      In [2]: titanic = pd.read_csv("data/titanic.csv")

      In [3]: titanic.head()
      Out[3]:
         PassengerId  Survived  Pclass  ...     Fare Cabin Embarked
      0            1         0       3  ...   7.2500   NaN        S
      1            2         1       1  ...  71.2833   C85        C
      2            3         1       3  ...   7.9250   NaN        S
      3            4         1       1  ...  53.1000  C123        S
      4            5         0       3  ...   8.0500   NaN        S

      [5 rows x 12 columns]

# How to calculate summary statistics

## Aggregating statistics

- What is the average age of the Titanic passengers?

      In [4]: titanic["Age"].mean()
      Out[4]: np.float64(29.69911764705882)

Different statistics are available and can be applied to columns with numerical data. Operations in general exclude missing data and operate across rows by default.

- What is the median age and ticket fare price of the Titanic passengers?

      In [5]: titanic[["Age", "Fare"]].median()
      Out[5]:
      Age     28.0000
      Fare    14.4542
      dtype: float64

  The statistic applied to multiple columns of a `DataFrame` (the selection of two columns returns a `DataFrame`, see the subset data tutorial) is calculated for each numeric column.

The aggregating statistic can be calculated for multiple columns at the same time. Remember the `describe` function from the first tutorial?

    In [6]: titanic[["Age", "Fare"]].describe()
    Out[6]:
                  Age        Fare
    count  714.000000  891.000000
    mean    29.699118   32.204208
    std     14.526497   49.693429
    min      0.420000    0.000000
    25%     20.125000    7.910400
    50%     28.000000   14.454200
    75%     38.000000   31.000000
    max     80.000000  512.329200

Instead of the predefined statistics, specific combinations of aggregating statistics for given columns can be defined using the `DataFrame.agg()` method:

    In [7]: titanic.agg(
       ...:     {
       ...:         "Age": ["min", "max", "median", "skew"],
       ...:         "Fare": ["min", "max", "median", "mean"],
       ...:     }
       ...: )
       ...:
    Out[7]:
                  Age        Fare
    min      0.420000    0.000000
    max     80.000000  512.329200
    median  28.000000   14.454200
    skew     0.389108         NaN
    mean          NaN   32.204208

To user guide

Details about descriptive statistics are provided in the user guide section on descriptive statistics.

## Aggregating statistics grouped by category

- What is the average age for male versus female Titanic passengers?

      In [8]: titanic[["Sex", "Age"]].groupby("Sex").mean()
      Out[8]:
                    Age
      Sex
      female  27.915709
      male    30.726645

  As our interest is the average age for each gender, a subselection on these two columns is made first: `titanic[["Sex", "Age"]]`. Next, the `groupby()` method is applied on the `Sex` column to make a group per category. The average age for each gender is calculated and returned.

Calculating a given statistic (e.g. `mean` age) for each category in a column (e.g. male/female in the `Sex` column) is a common pattern. The `groupby` method is used to support this type of operations. This fits in the more general `split-apply-combine` pattern:

- Split the data into groups

- Apply a function to each group independently

- Combine the results into a data structure

The apply and combine steps are typically done together in pandas.

In the previous example, we explicitly selected the 2 columns first. If not, the `mean` method is applied to each column containing numerical columns by passing `numeric_only=True`:

    In [9]: titanic.groupby("Sex").mean(numeric_only=True)
    Out[9]:
            PassengerId  Survived    Pclass  ...     SibSp     Parch       Fare
    Sex                                      ...
    female   431.028662  0.742038  2.159236  ...  0.694268  0.649682  44.479818
    male     454.147314  0.188908  2.389948  ...  0.429809  0.235702  25.523893

    [2 rows x 7 columns]

It does not make much sense to get the average value of the `Pclass`. If we are only interested in the average age for each gender, the selection of columns (square brackets `[]` as usual) is supported on the grouped data as well:

    In [10]: titanic.groupby("Sex")["Age"].mean()
    Out[10]:
    Sex
    female    27.915709
    male      30.726645
    Name: Age, dtype: float64

Note

The `Pclass` column contains numerical data but actually represents 3 categories (or factors) with respectively the labels ‘1’, ‘2’ and ‘3’. Calculating statistics on these does not make much sense. Therefore, pandas provides a `Categorical` data type to handle this type of data. More information is provided in the user guide Categorical data section.

- What is the mean ticket fare price for each of the sex and cabin class combinations?

      In [11]: titanic.groupby(["Sex", "Pclass"])["Fare"].mean()
      Out[11]:
      Sex     Pclass
      female  1         106.125798
              2          21.970121
              3          16.118810
      male    1          67.226127
              2          19.741782
              3          12.661633
      Name: Fare, dtype: float64

  Grouping can be done by multiple columns at the same time. Provide the column names as a list to the `groupby()` method.

To user guide

A full description on the split-apply-combine approach is provided in the user guide section on groupby operations.

## Count number of records by category

- What is the number of passengers in each of the cabin classes?

      In [12]: titanic["Pclass"].value_counts()
      Out[12]:
      Pclass
      3    491
      1    216
      2    184
      Name: count, dtype: int64

  The `value_counts()` method counts the number of records for each category in a column.

The function is a shortcut, it is actually a groupby operation in combination with counting the number of records within each group:

    In [13]: titanic.groupby("Pclass")["Pclass"].count()
    Out[13]:
    Pclass
    1    216
    2    184
    3    491
    Name: Pclass, dtype: int64

Note

Both `size` and `count` can be used in combination with `groupby`. Whereas `size` includes `NaN` values and just provides the number of rows (size of the table), `count` excludes the missing values. In the `value_counts` method, use the `dropna` argument to include or exclude the `NaN` values.

To user guide

The user guide has a dedicated section on `value_counts` , see the page on discretization.

#### REMEMBER

- Aggregation statistics can be calculated on entire columns or rows.

- `groupby` provides the power of the split-apply-combine pattern.

- `value_counts` is a convenient shortcut to count the number of entries in each category of a variable.

To user guide

A full description on the split-apply-combine approach is provided in the user guide pages about groupby operations.