DA Data Cleaning and Preparation 3

Question 1

What is a left join imputation strategy?

Accepted Answer

Filling NULLs from a reference lookup table via a left join

Answer

Joining a dataset to a reference table on a key and using matched values to fill NULLs is a practical imputation strategy in SQL-based data preparation.

Question 2

What is the difference between structured and unstructured data?

Accepted Answer

Structured data fits in predefined rows/columns; unstructured data lacks a fixed schema

Answer

Structured data fits neatly into tables (e.g., databases), while unstructured data (emails, images, text) requires processing before it can be analyzed.

Question 3

What is the purpose of a data dictionary?

Accepted Answer

Documenting the definitions, data types, and relationships of dataset fields

Answer

A data dictionary is a reference document that describes each field in a dataset — its name, type, allowed values, and meaning.

Question 4

What is the risk of removing rows with missing values (listwise deletion)?

Accepted Answer

It can introduce bias if missing data is not random

Answer

Listwise deletion can bias results if data is not missing completely at random, as systematically removing rows may skew the remaining sample.

Question 5

What is data reshaping in pandas (Python)?

Accepted Answer

Converting data between wide and long formats using melt or pivot

Answer

Reshaping transforms the layout of a DataFrame, such as using melt() to go from wide to long format or pivot() to do the reverse.

DA - Data Analyst Practice Test

DA - Data Analyst Practice Test

DA Data Cleaning and Preparation 3