fastai 2020 course-v4 Part1, lesson07

Follow Apr 29, 2020 · 14 mins read
fastai 2020 course-v4 Part1, lesson07
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Note of lesson7 : Deep Learning Part 1 of Spring 2020 at USF

Today we will study

  1. Collaborative Filtering
  2. Tabular modelling


Collaborative Filtering

Collaborative filtering from scratch

Last time issue: We are overfitting with just small batch</br> Capacity of model, normally reducing the parameter of model ends up very shallow model

Weight decay

a= 50 means just small change will give you big results.


weight decay means adding some valud to the gradients, so that it makes more shallow and less bumpy loss.

Be cautious, when you do statistical model, they lessen the parameter to evade overfitting but modern machine learning doesn’t do that, and add regularization

Creating our own Embedding module

Embedding is just indexing into an array

Normally layer is made from inhertancing of module

when you call torch and make tensor, they doesn’t have parameter, and you should define it using nn.parameter

def create_params(size):
    return nn.Parameter(torch.zeros(*size).normal_(0, 0.01))

class DotProductBias(Module):
    def __init__(self, n_users, n_movies, n_factors, y_range=(0,5.5)):
        self.user_factors = create_params([n_users, n_factors])
        self.user_bias = create_params([n_users])
        self.movie_factors = create_params([n_movies, n_factors])
        self.movie_bias = create_params([n_movies])
        self.y_range = y_range
    def forward(self, x):
        users = self.user_factors[x[:,0]]
        movies = self.movie_factors[x[:,1]]
        res = (users*movies).sum(dim=1)
        res += self.user_bias[x[:,0]] + self.movie_bias[x[:,1]]
        return sigmoid_range(res, *self.y_range)

pytorch has autograd so that we don’t have to calculate gradient manually

What’s our advantage making our own embedding instead of using pytorch one J: no advantage just implementing by yourself and easily learn concept.

Interpreting embeddings and biases

movie_bias = learn.model.movie_bias.squeeze()
idxs = movie_bias.argsort()[:5]
[dls.classes['title'][i] for i in idxs]

meaning of this, not just (#f)

we can use pca, and filter factors to 3, so we can select some kind of latent factors. 1

Using fastai.collab

learn = collab_learner(dls, n_factors=50, y_range=(0, 5.5))
learn.fit_one_cycle(5, 5e-3, wd=0.1)

you can get similar results using fastai

Embedding distance

Distance between two movies.

Let’s pick some movie and find distance, form one movie to every other movies.


(#f) J: visualization part is just what’s going in and..? sound quality is not good

Boot strapping a collaborative filtering

Deep Learning for collaborative filtering

the other way to do collab, we can concatenate the embeddings

(#q) can’t understand

get_emb_sz method, will give you the layer’s tensor size

learn = collab_learner(dls, use_nn=True, y_range=(0, 5.5), layers=[100,50])
learn.fit_one_cycle(5, 5e-3, wd=0.1)

if you use params use_nn=True, you can use concat version of collab

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Tabular modelling

Categorical embeddings

It’s been a not much time that tabular data is used for deep learning model.

Ex) German regions in embedding, if we draw embedding of the german city, it resembles actual german map

many kinds of information of the world can be represented using embedding

This is how google’s recommendation system works.

They use collab, which is concatenation of embeddingsgoogoo

Beyond Deep Learning

sometimes the other model, which is not deep learning models(classical model) do better than modern deep learning model

  1. Ensembles of decision trees (i.e. Random Forests and Gradient Boosting Machines), mainly for structured data (such as you might find in a database table at most companies)
  2. Multi-layered neural networks learnt with SGD (i.e. shallow and/or deep learning), mainly for unstructured data (such as audio, vision, and natural language)

try 1 first, and after that try 2.

How the decision tree ensenble works?

we usually scikit-learn library.

Refer this book

when you use y_range maximum as 5.5? J: because it uses sigmoid, which can’t reach to the max, min value 2

Do you recommend real-time service using decision-tree? for my own experience it was too slow. J:

The dataset

kaggle Competitions

Look at the data

At this point, a good next step is to handle ordinal columns and in this case, it is size of the product. we can make it as a categorical variables

Jeremy read this part, this it when I do review

Decision trees

It doesn’t require you special coding skill

This is some good example of feature engineering

Handling dates

Using TabularPandas and TabularProc

TabularProc - Tabular process, which is in-place function.

Categorify - make column with a numeric value, which is same with we saw at vocab.

And we should think about valiation, and at this case, making test set as random is not enough. because this is basically related to time.

See how test set is divided, and we can set the validation set as future time data.

Creating the decision tree

DecisionTree Regressor, something when we want to predict continuous variable.

Data augmentation for tabular data, do you have idea? J: Not sure, but will think about data semantics unordered, and ordered category, does distinguish between these? J: yes

you can use fastai function when drawing decision tree.

draw_tree(m, xs, size=7, leaves_parallel=True, precision=2)

look at diagram from top to bottom. it’s the best way.

basic way to do regression is predict the average.

leaf-node: not further trees when it is False

if you want to train further, we can choice leaf-note number bigger

m_rmse(m, xs, y)
> 0.0

means we made perfect model, but when we check validation model, it’s bigger than 0 - one reason is related to leaf-node

So we made our leaf-note to 25, and our result

do random tree (#f) ??

Categorical variables

related to categorical variables, we don’t have to do one-hot encode like using neural net! (we can make it, but there is no evidence it’s better)

What can I do with categorical missing value? J: (#f)

Random forests

(#q could not understand much)

bagging predictors

: “Bagging predictors is a method for generating multiple versions of a predictor and using these to get an aggregated predictor. The aggregation averages over the versions… The multiple versions are formed by making bootstrap replicates of the learning set and using these as new learning sets. Tests… show that bagging can give substantial gains in accuracy. The vital element is the instability of the prediction method. If perturbing the learning set can cause significant changes in the predictor constructed, then bagging can improve accuracy.”

random subset, when you average the error, it is zero???

Also randomly choose the subset of columns

Creating a random forest

def rf(xs, y, n_estimators=40, max_samples=200_000,
       max_features=0.5, min_samples_leaf=5, **kwargs):
    return RandomForestRegressor(n_jobs=-1, n_estimators=n_estimators,
        max_samples=max_samples, max_features=max_features,
        min_samples_leaf=min_samples_leaf, oob_score=True).fit(xs, y)

in general, when you increase the n_estimators, the accuracy enhances more

plt.plot([r_mse(preds[:i+1].mean(0), valid_y) for i in range(40)]);

we can set up our own methods, to see what’s going on. you just picked first element of the result, and show them.

but our evaluation get low. why?

Out-of-bag error

it’s not about time set, we don’t need validation set to do it.

Model interpretation

Tree variance for prediction confidence

How confident are we in our predictions using a particular row of data?

  • look at the standard deviation of the tree, how much did the tree were varied, and if we were very varied, it means we didn’t confident that much

Feature importance

For predicting with a particular row of data, what were the most important factors, and how did they influence that prediction?

  • m.feature_importance attribute

Removing low-importance variables

Which columns are the strongest predictors, which can we ignore?

remove the ignorable tree, and re train, accuracy is about to same, but the number of feature decreased. after

before we had this much features

It decreased!

Removing redundant features

Which columns are effectively redundant with each other, for purposes of prediction? How do predictions vary, as we vary these columns?

in this way we can get simpler and simpler model, remaining the accuracy

Partical dependence

best way is to draw histogram.

we can draw depends on each label, and see parti

what is said, how YearMade impact sales, if all other valud is equal?

What might be impact just mitigate specific variable?

is cluster doing something related to hierarchical J: yes

How to the feature importance values related to correlation? J: if you used to linear regression (#f) / Larry D. Here is an answer from Fra Pochetti: If 2 features are highly correlated their relative feature importance would be reduced compared to keeping just one of the two. Here why. A random forest selects features randomly at each split (in general). If 2 variables are correlated, they more or less carry the same signal wrt the dependent variable. Hence you can expect a tree to split on either of the 2 evenly. As an end result, your 2 features will have much less importance, just because they are carrying the same information. They hide each other. I generally remove correlated features even if it is not strictly needed, just to be able to uncover these kind of hidden relationships and spot truly important variables.

Data leakage

Tree interpreter

treeinterpreter module

So we take that one row of data, and put it through the first decision tree, looking to see what split is used at each point throughout the tree.

prediction[0], bias[0], contributions[0].sum()

Extrapolation and neural networks

Jeremy usually index using special value ‘None’, not unsqeeze.

random forest can’t extrapolate out side of train set, what it has seen, and this is big problem

Finding out of domain data

concatenating all of the independent variable,

and ask is this data from training set or validation set?

difference between valid data and train set,

and extrapolation will not happen if it’s neural net.

Using a neural network

In fastai, a tabular model is simply a model which takes columns of continuous or categorical data, and predicts a category (a classification model) or a continuous value (a regression model). Categorical independent variables are passed through an embedding, and concatenated, as we saw in the neural net we used for collaborative filtering, and then continuous variables are concatenated as well.

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It’s simple we can just do average.


another approach when we do ensemble,

this is pretty sensitive to hyper-parameters

with tabular, dropping the feature is better than regularization? J: (#f) J:

Combining embeddings with other methods

Entity embedding, which is EE

Conclusion: our advice for tabular modeling

slow regarding inference time, because they have to infer at every tree


Jiwon, here is an answer from Jona R on the forum: Not certain, but I think that “0” was not actually a choice that users could rate. It seemed to me like it was used as a placeholder for “no rating” in some of Jeremy’s models. That means that the lowest you would need to predict is “1”, which means that setting range 0 provides sufficient buffer.

  1. wasn’t it 3d? how can we draw it using 2d drawing? 

  2. why it does not use minimum to minus? like [-0.5, 5.5]