Production data is generated by the real environment where a deployed machine-learning model operates. Training data should resemble it closely; otherwise, strong test accuracy can hide poor real-world performance. section 6.1.2
When polished images fail in practice
A dish-recognition app trained on professional food photographs achieved more than 90% test accuracy but failed on mobile photos taken in ordinary restaurants. The training images did not represent the production data. Collecting and annotating images directly from the intended environment would better match deployment conditions. section 6.1.2
Use production data throughout the model lifecycle:
- Collect data directly from the deployment environment when possible, such as new IMDb reviews for an IMDb sentiment model or actual tweets for a tweet sentiment model. section 6.1.2
- If direct collection is impossible, understand the differences between training and production data and actively correct them. section 6.1.2
- After deployment, monitor new inputs, model performance, application interactions, and business outcomes; use audits or randomized A/B tests when appropriate. section 6.3.3
- Keep collecting and annotating data, especially difficult cases, because production characteristics can change and gradually reduce model relevance. section 6.3.4
“DEWEY DEFEATS TRUMAN”: a famous example of sampling bias
Sampling bias provides a visual warning about how collected data can misrepresent the cases encountered in operation. section 6.1.2

