After getting a feel for the overall goal of descriptive statistics, let's start by pinpointing the "center" of our data. When someone asks what a typical value in a dataset looks like, they're usually asking about its central tendency. We have three primary ways to measure this: the mean, the median, and the mode. Each offers a different perspective on what constitutes the "middle" or "most common" value, and understanding their differences is important for accurate data interpretation.

The Mean: The Familiar Average

The mean, specifically the arithmetic mean, is the most common measure of central tendency. It's calculated by summing all the values in a dataset and dividing by the number of values.

If we have a dataset with $n$ observations denoted as $x_1, x_2, ..., x_n$ , the sample mean, often represented by $\bar{x}$ (pronounced "x-bar"), is calculated as:

\bar{x} = \frac{1}{n} \sum_{i=1}^{n} x_i = \frac{x_1 + x_2 + \dots + x_n}{n}

For example, consider the dataset: {2, 3, 5, 6, 9}. The mean is $(2 + 3 + 5 + 6 + 9) / 5 = 25 / 5 = 5$ .

The mean incorporates every value in the dataset, which is one of its strengths. However, this also makes it sensitive to outliers or extreme values. A single very large or very small value can significantly pull the mean in its direction, potentially misrepresenting the center of the majority of the data.

Calculating the Mean in Python with Pandas:

Assuming you have your data in a Pandas Series or DataFrame column, calculating the mean is straightforward using the .mean() method.

import pandas as pd

data = pd.Series([2, 3, 5, 6, 9, 100]) # Added an outlier: 100

# Calculate the mean
mean_value = data.mean()

print(f"The dataset: {data.tolist()}")
print(f"The mean is: {mean_value}")
# Output:
# The dataset: [2, 3, 5, 6, 9, 100]
# The mean is: 20.833333333333332

Notice how the outlier (100) significantly increased the mean compared to the initial value of 5 when the dataset was {2, 3, 5, 6, 9}.

The Median: The Middle Ground

The median is the value that separates the higher half from the lower half of a dataset. To find it, you first need to sort the data in ascending order.

If the dataset has an odd number of observations ( $n$ is odd): The median is the middle value. Its position is $(n + 1) / 2$ .
If the dataset has an even number of observations ( $n$ is even): The median is the average of the two middle values. These values are at positions $n / 2$ and $(n / 2) + 1$ .

Let's revisit our examples:

Dataset: {2, 3, 5, 6, 9} (sorted)
- $n = 5$ (odd)
- Position = $(5 + 1) / 2 = 3$ .
- The 3rd value is 5. The median is 5.
Dataset: {2, 3, 5, 6, 9, 100} (sorted)
- $n = 6$ (even)
- Positions are $6 / 2 = 3$ and $(6 / 2) + 1 = 4$ .
- The 3rd value is 5, the 4th value is 6.
- The median is $(5 + 6) / 2 = 5.5$ .

The primary advantage of the median is its robustness to outliers. Extreme values have little to no impact on the median because it only depends on the middle value(s) after sorting. This makes it a better measure of central tendency for datasets that are skewed or contain significant outliers.

Calculating the Median in Python with Pandas:

Pandas provides the .median() method.

import pandas as pd

data_odd = pd.Series([2, 3, 5, 6, 9])
data_even = pd.Series([2, 3, 5, 6, 9, 100]) # With outlier

# Calculate the median
median_odd = data_odd.median()
median_even = data_even.median()

print(f"Dataset 1: {data_odd.tolist()}")
print(f"Median 1: {median_odd}")
print(f"\nDataset 2 (with outlier): {data_even.tolist()}")
print(f"Median 2: {median_even}")
# Output:
# Dataset 1: [2, 3, 5, 6, 9]
# Median 1: 5.0
#
# Dataset 2 (with outlier): [2, 3, 5, 6, 9, 100]
# Median 2: 5.5

Compare the mean (20.83) and median (5.5) for the dataset with the outlier. The median provides a much better sense of the "typical" value among the non-outlier numbers.

The Mode: The Most Frequent

The mode is the value that appears most frequently in a dataset. A dataset can have:

No mode: If all values occur with the same frequency (e.g., {1, 2, 3, 4, 5}).
One mode (unimodal): If one value appears more often than any other (e.g., {1, 2, 2, 3, 4}). The mode is 2.
Multiple modes (bimodal, multimodal): If two or more values share the highest frequency (e.g., {1, 1, 2, 3, 3, 4}). The modes are 1 and 3.

The mode is particularly useful for categorical data (e.g., finding the most common color or category) but can also be used with numerical data, especially discrete data. It's the only measure of central tendency that makes sense for nominal categorical data. Unlike the mean and median, the mode is not necessarily unique.

Calculating the Mode in Python with Pandas:

The .mode() method in Pandas returns a Series containing all modes (since there can be more than one).

import pandas as pd

data_unimodal = pd.Series([1, 2, 2, 3, 4, 4, 4, 5])
data_bimodal = pd.Series(['apple', 'banana', 'apple', 'orange', 'banana', 'banana'])

# Calculate the mode
mode_unimodal = data_unimodal.mode()
mode_bimodal = data_bimodal.mode()

print(f"Dataset 1: {data_unimodal.tolist()}")
print(f"Mode(s) 1: {mode_unimodal.tolist()}")
print(f"\nDataset 2: {data_bimodal.tolist()}")
print(f"Mode(s) 2: {mode_bimodal.tolist()}")
# Output:
# Dataset 1: [1, 2, 2, 3, 4, 4, 4, 5]
# Mode(s) 1: [4]
#
# Dataset 2: ['apple', 'banana', 'apple', 'orange', 'banana', 'banana']
# Mode(s) 2: ['banana']

Mean, Median, or Mode: Which to Use?

The choice depends heavily on the nature of your data and what you want to communicate:

Mean: Good for symmetric distributions without significant outliers. It uses all data points.
Median: Preferred for skewed distributions or data with outliers, as it's more robust and represents the true middle point.
Mode: Best for categorical data and useful for identifying the most common value(s) in numerical data.

The relationship between the mean, median, and mode can also offer insights into the skewness of the distribution:

Symmetric Distribution: Mean ≈ Median ≈ Mode.
Right-Skewed Distribution (Positively Skewed): Mean > Median > Mode. The tail points to the right, and large values pull the mean upwards.
Left-Skewed Distribution (Negatively Skewed): Mean < Median < Mode. The tail points to the left, and small values pull the mean downwards.

Consider this visualization showing approximate positions on different distribution shapes:

Approximate positions of Mean (dashed line), Median (dotted line), and Mode (peak frequency) for symmetric, right-skewed, and left-skewed distributions. In skewed distributions, the median typically lies between the mode and the mean.

Understanding the mean, median, and mode provides a first crucial step in summarizing your data. They tell you where the data tends to cluster, but they don't tell the whole story. Next, we'll explore how to measure how spread out the data is around this central point.