A learning algorithm usually performs better on the training set than on the dev set, so the dev-error curve usually lies above the training-error curve.

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If one only looks at the dev-error curve, it can be hard to extrapolate and predict exactly where the curve will go with more data. Training error is an additional plot that can help estimate the impact of adding more data.

Training Error Plot for Estimating the Effect of More Data

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Machine Learning Yearning (Deeplearning.ai)

As training-set size grows, dev and test error should decrease, but training-set error usually increases. With very small training sets, an algorithm can often memorize the examples and get near-zero training error; with larger and more ambiguous or mislabeled training sets, perfectly fitting every example becomes harder.

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