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Qiita 機械学習Published: Oct 5, 2026, 10:00 JST

99% accuracy, zero fraud caught: AWS guide warns on skewed data

99% accuracy, zero fraud caught: AWS guide warns on skewed data

3 Key Points

  1. What happened

    In the AWS Certified AI Practitioner guide, the author uses a fraud example: among 1,000 transactions with only 10 fraudulent, predicting everything as legitimate still gives 99% accuracy while catching none.

  2. Why it matters

    The example shows that when fraudulent cases are rare, a model can score 99% accuracy without catching a single one, so accuracy alone can be misleading in such skewed data.

  3. What to watch

    The guide adds that in skewed-data cases, metrics beyond accuracy — precision, recall, and F1 — should also be checked, so the test is whether teams evaluating fraud or anomaly detection adopt those additional metrics.

WHO IT HITSThis lands on non-engineers preparing for the AWS Certified AI Practitioner exam, such as business analysts and product managers, who need to understand why accuracy alone is not enough when data is heavily skewed.

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Context & Analysis

The AWS Certified AI Practitioner guide is aimed at non-engineers who need to clear the exam, which explains why it spends so much space on concrete failure modes rather than math. The fraud example is the pivot: it shows that a 99% accuracy score can completely hide the fact that a model catches zero fraud. From there, the guide introduces precision, recall, and F1 so readers can see why skewed data demands more than one number.

The same theme runs through the generative-AI section. The guide lists risks like hallucinations, prompt injection, and leakage of sensitive information, and names Amazon Bedrock Guardrails as the service for filtering and masking. It is a menu of controls, not a deep dive, so the real test is whether readers can match each risk to the right tool when they practice questions.

In that sense, the guide's value hinges on how well it connects metrics and controls to exam scenarios. Precise definitions matter less than being able to recognize when a metric misleads or which guardrail applies, which is likely why it leans so heavily on worked examples.

FAQ
What is the main risk of relying only on accuracy in machine learning?
In skewed data, a model can achieve 99% accuracy while catching none of the fraudulent transactions, because the vast majority of cases are normal. The guide says you need to check other metrics like precision, recall, and F1.
What AWS service does the guide recommend for filtering harmful content and blocking personal information in generative AI?
Amazon Bedrock Guardrails. The guide says it can filter harmful content, block or mask personal information, and detect prompt attacks.
What is the difference between overfitting and underfitting, according to the guide?
Overfitting means the model fits the training data too closely and fails on new data, while underfitting means it cannot capture the patterns well enough and performs poorly even on training data.
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