Hard parameter sharing is the most commonly used approach to MTL in neural networks. It is generally applied by sharing the hidden layers between all tasks, while keeping several task-specific output layers as can be seen in the figure below.


Hard Parameter Sharing

In soft parameter sharing, each task has its own model with its own parameters. The distance between the parameters of the model is then regularized in order to encourage the parameters to be similar, as evidenced in figure below.

Soft Parameter Sharing

In the context of Deep Learning, multi-task learning is typically done with either hard or soft parameter sharing of hidden layers.


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In Machine Learning (ML), we typically care about optimizing for a particular metric. While we can generally achieve acceptable performance this way, by only focusing on a single task, we ignore information from training signals of related tasks that might help us do even better on the metric we care about. By contrast, MTL improves generalization by leveraging the domain-specific information contained in the training signals of related tasks. 

Multi-task Learning

Ruder, S. (n.d.). An Overview of Multi-Task Learning in Deep Neural Networks *. Retrieved May 22, 2022, from https://arxiv-download.xixiaoyao.cn/pdf/1706.05098.pdf

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Multi-Task Learning in Deep Neural Networks

1. Biological perspective: we can see multi-task learning as being inspired by human learning. For learning new tasks, we often apply the knowledge we have acquired by learning related tasks. 


2. Pedagogical perspective: we often learn tasks that provide us with the necessary skills to master more complex techniques first.


3. Machine learning perspective: We can view multi-task learning as a form of inductive transfer. Inductive transfer can help improve a model by introducing an inductive bias, which causes a model to prefer some hypotheses over others.


Motivations of MTL

Two MTL Methods for Deep Learning


For all examples, we will assume that we have two related tasks A and B, which rely on a common hidden layer representation F.

Reasons Why MTL Works

There are two main ideas that have been pervasive throughout the history of multi-task learning: enforcing sparsity across tasks through norm regularization; and modeling the relationships between tasks.


MTL in Non-neural Models

While many recent Deep Learning approaches have more or less used multi-task learning as part of their model , they all employed either hard or soft parameter sharing, and sometimes both.


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