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I am reading Goodfellow et al Deeplearning Book. I found it difficult to understand the difference between the definition of the hypothesis space and representation capacity of a model.

In Chapter 5, it is written about hypothesis space:

One way to control the capacity of a learning algorithm is by choosing its hypothesis space, the set of functions that the learning algorithm is allowed to select as being the solution.

And about representational capacity:

The model specifies which family of functions the learning algorithm can choose from when varying the parameters in order to reduce a training objective. This is called the representational capacity of the model.

If we take the linear regression model as an example and allow our output $y$ to takes polynomial inputs, I understand the hypothesis space as the ensemble of quadratic functions taking input $x$, i.e $y = a_0 + a_1x + a_2x^2$.

How is it different from the definition of the representational capacity, where parameters are $a_0$, $a_1$ and $a_2$?

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Consider a target function $f: x \mapsto f(x)$.

A hypothesis refers to an approximation of $f$. A hypothesis space refers to the set of possible approximations that an algorithm can create for $f$. The hypothesis space consists of the set of functions the model is limited to learn. For instance, linear regression can be limited to linear functions as its hypothesis space, or it can be expanded to learn polynomials.

The representational capacity of a model determines the flexibility of it, its ability to fit a variety of functions (i.e. which functions the model is able to learn), at the same. It specifies the family of functions the learning algorithm can choose from.

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    $\begingroup$ Does it mean that the set of functions described by the representational capacity is strictly included in the hypothesis space ? By definition, is it possible to have functions in the hypothesis space NOT described in the representational capacity ? $\endgroup$ – Qwarzix Aug 23 '18 at 8:43
  • $\begingroup$ It's still pretty confusing to me. Most sources say that a "model" is an instance (after execution/training on data) of a "learning algorithm". How, then, can a model specify the family of functions the learning algorithm can choose from? It doesn't make sense to me. The authors of the book should've explained these concepts in more depth. $\endgroup$ – Talendar Oct 9 at 13:09
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A hypothesis space is defined as the set of functions $\mathcal H$ that can be chosen by a learning algorithm to minimize loss (in general).

$$\mathcal H = \{h_1, h_2,....h_n\}$$

The hypothesis class can be finite or infinite, for example a discrete set of shapes to encircle certain portion of the input space is a finite hypothesis space, whereas hpyothesis space of parametrized functions like neural nets and linear regressors are infinite.

Although the term representational capacity is not in the vogue a rough definition woukd be: The representational capacity of a model, is the ability of its hypothesis space to approximate a complex function, with 0 error, which can only be approximated by infinitely many hypothesis spaces whose representational capacity is equal to or exceed the representational capacity required to approximate the complex function.

The most popular measure of representational capacity is the $\mathcal V$ $\mathcal C$ Dimension of a model. The upper bound for VC dimension ($d$) of a model is: $$d \leq \log_2| \mathcal H|$$ where $|H|$ is the cardinality of the set of hypothesis space.

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A hypothesis space/class is the set of functions that the learning algorithm considers when picking one function to minimize some risk/loss functional.

The capacity of a hypothesis space is a number or bound that quantifies the size (or richness) of the hypothesis space, i.e. the number (and type) of functions that can be represented by the hypothesis space. So a hypothesis space has a capacity. The two most famous measures of capacity are VC dimension and Rademacher complexity.

In other words, the hypothesis class is the object and the capacity is a property (that can be measured or quantified) of this object, but there is not a big difference between hypothesis class and its capacity, in the sense that a hypothesis class naturally defines a capacity, but two (different) hypothesis classes could have the same capacity.

Note that representational capacity (not capacity, which is common!) is not a standard term in computational learning theory, while hypothesis space/class is commonly used. For example, this famous book on machine learning and learning theory uses the term hypothesis class in many places, but it never uses the term representational capacity.

Your book's definition of representational capacity is bad, in my opinion, if representational capacity is supposed to be a synonym for capacity, given that that definition also coincides with the definition of hypothesis class, so your confusion is understandable.

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    $\begingroup$ I agree with you. The authors of the book should've explained these concepts in more depth. Most sources say that a "model" is an instance (after execution/training on data) of a "learning algorithm". How, then, can a model specify the family of functions the learning algorithm can choose from? Also, as you pointed out, the definition of the terms "hypothesis space" and "representational capacity" given by the authors are practically the same, although they use the terms as if they represent different concepts. $\endgroup$ – Talendar Oct 9 at 13:18

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