Mean squared error (MSE) measures the average squared difference between predictions and target numerical values. During neural-network training, it turns errors across a batch or training set into one cost that the optimizer tries to minimize. section 5.3
MSE is useful for understanding training because it connects several practical ideas:
- A lower position on the error surface represents weights that produce smaller average errors across the training data. section 5.3
- MSE suits regression problems, whereas binary classification more commonly uses binary cross-entropy. section 5.2.5
- The training objective aggregates errors across examples rather than minimizing the error of only one example. section 5.3
Reading the error surface
Imagine plotting possible weight values horizontally and MSE vertically. The lowest point is the weight configuration with the smallest average prediction error for the training set; gradient-based optimization moves toward that region. section 5.3
Figure 5.6 Convex error curve
Use the convex error-curve figure to visualize how changing weights changes the aggregate error and where the minimum lies. section 5.3












