# Review — There Are Many Consistent Explanations of Unlabeled Data: Why You Should Average

## SWA & Fast SWA, Improving Π-Model & Mean Teacher

@ Medium)

There Are Many Consistent Explanations of Unlabeled Data: Why You Should AverageSWA & Fast SWA, by Cornell University2019 ICLR, Over 100 Citations(

Semi-Supervised Learning, Image Classification

**Stochastic Weight Averaging (SWA)**is proposed, which**averages weight along the trajectory of SGD**with a modified learning rate schedule.**Fast-SWA**is further proposed, which**accelerates convergence by averaging multiple points within each cycle**of a cyclical learning rate schedule.

# Outline

**Conventional****Π-Model****&****Mean Teacher****(MT)****Proposed Stochastic Weight Averaging (SWA) & Fast SWA****Experimental Results**

**1. Conventional **Π-Model** & **Mean Teacher** (MT)**

## 1.1. Consistency Loss

- In the semi-supervised setting, we have access to
**labeled data**={(*DL**xLi*,*yLi*)} for*i*from 1 to*NL*, and**unlabeled data**={*DU**xUi*} for*i*=1 to*NU*. - Given
**two perturbed inputs**of*x*’,*x*’’*x*and the perturbed weights*w*’*f*and*w*’*g*, the**consistency loss**penalizes the difference between the student’s predicted probabilities*f*(*x*’,*w*’*f*) and the teacher’s*g*(*x*’’,*w*’*g*). - This loss is typically the
**Mean Squared Error**or**KL divergence**:

- The
**total loss**used to train the model can be written as:

- where for
**classification***LCE***cross entropy**between the model predictions and supervised training labels. The parameter*λ*>0 controls the relative importance of the consistency term in the overall loss.

## 1.2. Π-Model

- Π-Model uses the
**student model***f*as its own teacher. - But the
**data (input) is perturbed**by random translations, crops, flips and additive Gaussian noise. Binary**dropout**is used for**weight perturbation**.

## 1.3. Mean Teacher (MT)

**The teacher weights***wg*are the exponential moving average (EMA) of the student weights*wf*:

- where the decay rate
*α*is usually set between 0.9 and 0.999.

**2. Proposed Stochastic Weight Averaging (SWA) & Fast SWA**

## 2.1. Cyclical Schedule

- Stochastic Weight Averaging (SWA) is a recent approach by Izmailov et al., 2018, which is based on
**averaging weights traversed by SGD with a modified learning rate schedule.** - For the first
*l*≤*l*0 epochs, the network is pre-trained using the cosine annealing schedule. **After***l*epochs, a cyclical schedule is used, repeating the learning rates from epochs [*l*-*c*,*l*], where*c*is the cycle length.

## 2.2. SWA

**Left (green dot)**:**SWA collects the networks corresponding to the minimum values of the learning rate and averages their weights.**The model with the averaged weights*wSWA*is then used to make predictions.- SWA is
**applied to the student network**both for the Π-Model and Mean Teacher model. - However,
**SWA updates the average weights only once per cycle**, which means that**many additional training epochs are needed**in order to collect enough weights for averaging.

## 2.3. Fast SWA

**Left (red dot)**: A modification of SWA that averages networks corresponding to every*k*<*c*epochs starting from epoch*l*-*c*.**Average multiple weights are obtained within a single epoch**setting*k*<1.

**3. Experimental Results**

For all quantities of labeled data,

fast-SWA substantially improves test accuracyin both architectures.

The above table shows the summary that fast-SWA can significantly improve the performance of both the Π-Model and Mean Teacher Model.

Please feel free to read the paper for more detailed results if interested.

## Reference

[2019 ICLR] [SWA & Fast SWA]

There Are Many Consistent Explanations of Unlabeled Data: Why You Should Average

## Pretraining or Weakly/Semi-Supervised Learning

**2004 **[Entropy Minimization, EntMin] **2013** [Pseudo-Label (PL)] **2015** [Ladder Network, Γ-Model] **2016 **[Sajjadi NIPS’16] **2017** [Mean Teacher] [PATE & PATE-G] [Π-Model, Temporal Ensembling] **2018 **[WSL] [Oliver NeurIPS’18] **2019** [VAT] [Billion-Scale] [Label Propagation] [Rethinking ImageNet Pre-training] [MixMatch] [SWA & Fast SWA] **2020 **[BiT] [Noisy Student] [SimCLRv2]