Test Data. Algorithm 1 (solid line) modeled the training data perfectly, but has very high error on the test data. Algorithm 2 had a higher error on the training data, but models the test data with a reasonably low error. Algorithm 1 is an example of overfitting. Algorithm 2 is a much better real-world predictor.
	Part of: Thessen A (2016) Adoption of Machine Learning Techniques in Ecology and Earth Science. One Ecosystem 1: e8621. https://doi.org/10.3897/oneeco.1.e8621