Aggregation functions (sum, mean, std) in Data Analysis Python - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of sum aggregation on DataFrame column

What is the output of the following code snippet?

Data Analysis Python

import pandas as pd

df = pd.DataFrame({'values': [10, 20, 30, 40]})
result = df['values'].sum()
print(result)

A 40

B TypeError

C 100

D 25

💡 Hint

Sum adds all numbers in the column.

The sum of 10 + 20 + 30 + 40 equals 100.

❓ data_output

intermediate

Mean value of grouped data

Given the DataFrame below, what is the mean value of group 'B'?

Data Analysis Python

import pandas as pd

df = pd.DataFrame({'group': ['A', 'B', 'B', 'A', 'B'], 'score': [5, 10, 15, 20, 25]})
mean_b = df.groupby('group')['score'].mean().loc['B']
print(mean_b)

A 16.666666666666668

B 15

C 10

D 20

💡 Hint

Mean is the sum divided by count for group 'B'.

Group 'B' has scores 10, 15, and 25. Their mean is (10+15+25)/3 = 16.666666666666668.

❓ Predict Output

advanced

Standard deviation calculation on a list

What is the output of this code?

Data Analysis Python

import numpy as np

values = [2, 4, 4, 4, 5, 5, 7, 9]
std_dev = np.std(values, ddof=1)
print(round(std_dev, 2))

A 2.14

B 2.00

C 2.31

D 1.96

💡 Hint

ddof=1 means sample standard deviation.

Sample standard deviation of the list is approximately 2.14 after rounding.

❓ visualization

advanced

Interpreting aggregation results in a bar chart

You have this code that plots mean scores by category. What does the bar height for category 'X' represent?

Data Analysis Python

import pandas as pd
import matplotlib.pyplot as plt

df = pd.DataFrame({'category': ['X', 'Y', 'X', 'Y', 'X'], 'score': [3, 7, 5, 9, 4]})
mean_scores = df.groupby('category')['score'].mean()
mean_scores.plot(kind='bar')
plt.show()

A The total sum of scores in category 'X'

B The count of entries in category 'X'

C The highest score recorded in category 'X'

D The average score of all entries in category 'X'

💡 Hint

The code uses mean aggregation.

The bar height shows the mean (average) score for category 'X', calculated from scores 3, 5, and 4.

🧠 Conceptual

expert

Understanding difference between population and sample std deviation

Which statement correctly explains the difference between population and sample standard deviation calculations?

A Population std deviation divides by (n-1), sample divides by n

B Sample std deviation divides by (n-1) to correct bias, population divides by n

C Both divide by n but sample uses a different formula for variance

D Sample std deviation always gives smaller values than population std deviation

💡 Hint

Think about degrees of freedom in statistics.

Sample std deviation uses (n-1) in denominator to correct bias when estimating from a sample; population uses n.