DA0-002 Data Acquisition and Preparation Practice Question
A data team is integrating customer data from three sources. After joining, they find that the count of unique customers is lower than expected. What is the most likely cause?
⚠ Common exam trap
Many exam-takers assume missing values or duplicates are the primary cause of a lower unique count, but CompTIA Data+ tests the nuance that inconsistent key definitions—not missing data—are the most common reason for unexpected join results in multi-source integration scenarios.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Inconsistent key definitions.
When joining customer data from multiple sources, inconsistent key definitions (e.g., one source uses integer IDs while another uses string IDs, or different formats like 'CUST-001' vs '1001') cause the join to fail to match records that actually represent the same customer. This results in fewer unique customers than expected because the join treats mismatched keys as different entities, effectively dropping or misaligning records. The data team likely used an inner join or a left join that only retains matches based on exact key equality, so any key inconsistency reduces the count of matched unique customers.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
Inconsistent key definitions.
Why this is correct
Mismatched key formats cause join failures, reducing matches.
- ✗
Missing values in join keys.
Why it's wrong here
Missing keys would cause rows to be dropped, potentially reducing count, but inconsistent definitions are more common and systemic.
- ✗
Data truncation during transfer.
Why it's wrong here
Truncation may lose data but not specifically reduce unique count.
- ✗
Duplicate entries across sources.
Why it's wrong here
Duplicates would increase the count of unique customers, not decrease.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This DA0-002 practice question is part of Courseiva's free CompTIA certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the DA0-002 exam.