Quick Answer
The correct first step is to prune the decision tree to reduce complexity. This is because a 100% training accuracy paired with a drastically lower test accuracy (55%) is the classic hallmark of overfitting, where the model has memorized noise and specific patterns in the training data rather than learning generalizable rules. Pruning, such as limiting the maximum depth or setting a minimum samples per leaf, directly constrains the tree’s growth, forcing it to capture only the most significant splits and thus improving generalization. On the AWS Certified Machine Learning Specialty MLS-C01 exam, this scenario tests your ability to diagnose overfitting and prioritize the most immediate fix; a common trap is to suggest adding more data or features, but those are secondary actions or can worsen the problem. Remember the memory tip: “When your tree is too deep, prune it before you leap”—always address model complexity first before considering data augmentation or evaluation techniques.
This MLS-C01 practice question tests your understanding of exploratory data analysis. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
A data scientist is analyzing a dataset for a binary classification problem. The dataset has 10,000 samples and 200 features. After splitting into training (80%) and test (20%), the data scientist trains a decision tree classifier and achieves 100% accuracy on the training set but only 55% on the test set. Which step should the data scientist take first to address this issue?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "first"
Why it matters: Order matters here. You are being tested on which action comes before the others — not which action is generally useful.
Use cross-validation to evaluate model performance
Why wrong: Why C is wrong
Collect more training data
Why wrong: Why B is wrong
Add more features to the model
Why wrong: Why A is wrong
Prune the decision tree to reduce complexity
Why D is correct
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
Prune the decision tree to reduce complexity
Option D is correct because the large discrepancy between training and test accuracy indicates overfitting, and pruning the decision tree (e.g., limiting max_depth) reduces overfitting. Option A is wrong because more features may worsen overfitting. Option B is wrong because more data may help but is not the first step; also data is limited. Option C is wrong because cross-validation is a technique to evaluate model performance but does not directly fix overfitting; pruning does.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
For each option: why learners choose it and why it is or isn't the right answer here.
Use cross-validation to evaluate model performance
Why it's wrong here
Why C is wrong
Collect more training data
Why it's wrong here
Why B is wrong
Add more features to the model
Why it's wrong here
Why A is wrong
Prune the decision tree to reduce complexity
Why this is correct
Why D is correct
Clue confirmation
The clue word "first" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
Common exam traps
Many certification questions include familiar terms but test a specific constraint. Read the exact wording before choosing an answer that is generally true but wrong for this case.
Detailed technical explanation
This question should be treated as a scenario, not a definition check. Identify the problem, the constraint and the best action. Then compare each option against those facts.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
A cloud solutions architect for a retail company is evaluating services for a new workload. The correct answer here reflects best practice for the specific scenario described — not a general cloud recommendation. Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option. Cloud exam questions reward reading the constraint carefully: the same technology can be right or wrong depending on the use case.
What to study next
Identify which MLS-C01 exam domain this question belongs to, then review the specific concept being tested. Practise related questions in that domain and focus on understanding why each wrong answer is tempting — not just why the correct answer is right.
Exploratory Data Analysis — study guide chapter
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FAQ
Exploratory Data Analysis — This question tests Exploratory Data Analysis — Read the scenario before looking for a memorised answer..
The correct answer is: Prune the decision tree to reduce complexity — Option D is correct because the large discrepancy between training and test accuracy indicates overfitting, and pruning the decision tree (e.g., limiting max_depth) reduces overfitting. Option A is wrong because more features may worsen overfitting. Option B is wrong because more data may help but is not the first step; also data is limited. Option C is wrong because cross-validation is a technique to evaluate model performance but does not directly fix overfitting; pruning does.
Identify which MLS-C01 exam domain this question belongs to, then review the specific concept being tested. Practise related questions in that domain and focus on understanding why each wrong answer is tempting — not just why the correct answer is right.
Yes — watch for: "first". Order matters here. You are being tested on which action comes before the others — not which action is generally useful.
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 20, 2026
This MLS-C01 practice question is part of Courseiva's free Amazon Web Services 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 MLS-C01 exam.
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