Dropout is a regularization method for overfitting neural networks. During training, it randomly sets selected perceptron outputs to zero, preventing the network from depending too heavily on a small group of units. section 9.0 section 9.3.6
For unit i in layer l, let δᵢ be a binary mask: δᵢ = 1 with probability pᵢ and δᵢ = 0 with probability 1 − pᵢ. The masked activation is therefore δᵢaᵢ. Dropout is enabled during training and disabled during inference. section 9.3.6
The mask discourages co-adaptation, where correlated units repeatedly produce similar outputs. Because any unit may disappear in a training iteration, learning spreads across multiple units, producing behavior analogous to L2 regularization. section 9.3.6
A three-node layer as an ensemble
A three-node layer has 2³ = 8 possible on/off configurations. Each configuration acts as a subnetwork, with its probability determined by the individual keep probabilities; for example, all three units being active has probability p₀p₁p₂. section 9.3.6











