# Why WeightWatcher Works

I am frequently asked, why does weightwatcher work ?

The weightwatcher tool uses power law fits to model the eigenvalue density $\rho(\lambda)$ of weight matrices of any Deep Neural Network (DNN).

$\rho(\lambda)\sim \lambda^{-\alpha},\;\;\;\lambda\in[\lambda_{min},\lambda_{max}]$

The average power-law exponent $\langle\alpha\rangle$ is remarkably well correlated with the test accuracy when changing the number of layers and/or fine-tuning the hyperparameters. In our latest paper, we demonstrate this using a metaanalysis on hundreds of pre-trained models. This begs the question:

Why can we model the weight matrices of DNNs using power law fits ?

In theoretical chemistry and physics, we know that strongly correlated, complex systems frequently display power laws.

In many machine learning and deep learning models, the correlations also display heavy / power law tails. After all, the whole point of learning is to learn the correlations in the data. Be it a simple clustering algorithm, or a very fancy Deep Neural Network, we want to find the most strongly correlated parts to describe our data and make predictions.

For example, in strongly correlated systems, if you place an electron in a random potential, it will show a transition from delocalized to localized states, and the spectral density will display power law tails. This is called Anderson Localization, and Anderson won the Nobel Prize in Physics for this in 1977. In the early 90s, Cizeau and Bouchaud argued that a Wigner-Levy matrix will show a similar localization transition, and since then have modeled the correlation matrices in finance using their variant of heavy tailed random matrix theory (RMT). Even today this is still an area of active research in mathematics and in finance.

In my earlier days as a scientist, I worked on strongly correlated multi-reference ab initio methods for quantum chemistry. Here, the trick is to find the right correlated subspace to get a good low order description. I believe, in machine learning, the same issues arise. For this reason, I also model the correlation matrices in Deep Neural Networks $\mathbf{X}=\mathbf{W}^{T}\mathbf{W}$ using heavy tailed RMT.

Here I will show that the simplest machine learning model, Latent Semantic Analysis (LSA), shows a localization transition, and that this can be used to identify and characterize the heavy tail of the LSA correlation matrix.

##### Latent Semantic Analysis: a simple example of power law correlations

Take Latent Semantic Analysis (LSA). How do we select the Latent Space? We need to select the top-K components of the TF-IDF Term-Document matrix $\mathbf{M}$. I believe this can be done by selecting those K eigenvalues that best fit a power law. Here is an example, using the scikit-learn 20newsgroups data:

We call ths plot the Empirical Spectral Density (ESD). This is just a histogram plot, on a log scale, of the eigenvalues $\log_{10}\; \lambda$ of the TF-IDF correlation matrix $\mathbf{X}$. The correlation matrix is the square of the TF-IDF matrix

$\mathbf{X}=\mathbf{M^{T}}\mathbf{M}$

and the eigenvalues of $\mathbf{X}$ are the singular values of $\mathbf{M}$, squared: $\lambda_{i}=\sigma_{i}^{2}$.

We fit the eigenvalues to a power law (PL) using the python powerlaw package, which implements a standard MLE estimator.

fit = powerlaw.fit(eigenvalues)

The fit selects the optimal xmin=$\lambda_{min}$ using a brute force search, and returns the best PL exponent $\alpha$, and the quality of the fit $D$ (the KS-distance). The orange line displays the start of the power law tail, which contains the most strongly correlated eigenpairs.

We can evaluate the quality of the PL fit by comparing the ESD and the actual fit on log-log plot.

The blue solid line is the ESD on a log-log scale (or the PDF), and the blue dotted line is the PL fit. (The red solid line is the empirical CDF, and the red dotted line the fit). The PDF (blue) shows a very good linear fit up, except perhaps for largest eigenvalues, $\lambda\lesssim\lambda_{max}$. Likewise, the CDF (red) shows a very good fit, up until the end of the tail. This is typical of power-law fits on real-world data and is usually best described as a Truncated Power Law (TPL), with some noise in the very far tail *(more on the noise in a future post). And the reported KS-distance $D=0.008$, which is exceptional.

We can get even more insight into the quality of the fit by examining how the PL method selected xmin$=\lambda_{min}$, the start of the PL. Below, we plot the KS-distance for each possible choice of xmin:

The optimization landscape is convex, with a clear global minimum at the orange line, which occurs at the $\lambda_{547}=\lambda{min}$. That is, there are 547 eigenpairs in the tail of the ESD displaying strong power-law behavior.

To form the Latent space, we select these largest 547 eigenpairs, to the right of the orange line, the start of the (truncated) power-law fit

#### The Localization Transition

To identify the localization transition in LSA, we can plot localization ratios $\mathcal{L}(\mathbf{v})$ in the same way, where the localization is defined as in our first paper:

$\mathcal{L}(\mathbf{v})=\dfrac{\Vert\mathbf{v}|\Vert_{1}}{\Vert\mathbf{v}|\Vert_{\infty}}$

def localization_ratio(v):
return np.linalg.norm(v, ord=1) / np.linalg.norm(v, ord=np.inf)

We see that get an elbow curve, and the eigenvalue cutoff appears just to the right of the ‘elbow’:

Other methods include looking scree plots or even just the sorted eigenvalues themselves.

##### Comparison with other K-selection methods

Typically, in unsupervised learning, one selects the top-K clusters, eigenpairs, etc. by looking at some so-called ‘elbow curve’, and identifying the K at the inflection point. We can make these plots too. A classic way is to plot the explained variance per eigenpair:

We see that the power-law $\lambda_{min}$, the orange line, occurs just to the right of the inflection point. So these two methods give similar results. No other method provides a theoretically well-defined way, however, of selecting the K components.

##### Conclusion

I suspect that in these strongly correlated systems, the power law behavior really kicks it right at / before these inflection points. So we can find the optimal low-rank approximation to these strongly correlated weight matrices by finding that subspace where the correlations follow a power-law / truncated power law distribution. Moreover, we can detect, and characterize these correlations, by both the power-law exponent $\alpha$, and the quality of the fit D.

And AFAIK, this has never been suggested before.