MS-DS Master of Data science Master of Data science Data Wrangling and Preprocessing 1

Question 1

Which pandas function is used to detect missing values in a DataFrame, returning a boolean mask?

Accepted Answer

df.isnull()

Answer

df.isnull() returns a DataFrame of the same shape with True where values are NaN and False elsewhere, making it the standard method for detecting missing data.

Question 2

What is the main difference between StandardScaler and MinMaxScaler in scikit-learn?

Accepted Answer

StandardScaler transforms features to zero mean and unit variance; MinMaxScaler scales features to a fixed range like [0,1]

Answer

StandardScaler centers data by subtracting the mean and dividing by standard deviation, producing zero mean and unit variance. MinMaxScaler maps values to a bounded range (default [0,1]) by subtracting the minimum and dividing by the range.

Question 3

In pandas, what is the effect of setting the parameter how='outer' in pd.merge()?

Accepted Answer

It returns all rows from both DataFrames, filling NaN where there is no match

Answer

An outer join returns the union of keys from both DataFrames. Where a key exists in one DataFrame but not the other, the missing columns are filled with NaN.

Question 4

Which technique is best suited for imputing missing values in a numerical column when the data is believed to be missing at random and the column has a moderate skew?

Accepted Answer

Median imputation

Answer

Median imputation is preferred over mean imputation when data is skewed because the median is robust to outliers and better represents the central tendency of a skewed distribution.

Question 5

What does the pandas groupby() method return before an aggregation function is applied?

Accepted Answer

A DataFrameGroupBy object

Answer

df.groupby() returns a DataFrameGroupBy object, which is a lazy grouping structure. The actual computation only occurs when an aggregation function such as .sum(), .mean(), or .agg() is called on it.

MS-DS Master of Data science Practice Test

MS-DS Master of Data science Practice Test

MS-DS Master of Data science Master of Data science Data Wrangling and Preprocessing 1