# Maggy Ablation: Feature and Model Ablation for the Titanic Dataset

Last Updated: 2020/04/06 Created: 2019/10/14

In this notebook, we demonstrate Maggy’s Ablation API, while using a TensorFlow Keras Sequential model trained on the Titanic Dataset. To be able to follow along, make sure you have the Titanic training dataset registered on your Project’s Feature Store, as explained in this example notebook.

## Wait … What is an Ablation Study?

An Ablation Study, in medical and psychological research, is a research method in which the roles and functions of an organ, tissue, or any part of a living organism, is examined through its surgical removal and observing the behaviour of the organism in its absence. This method, also known as experimental ablation, was pioneered by the French physiologist Marie Jean Pierre Flourens in the early nineteenth century. Flourens would perform ablative brain surgeries on animals, removing different parts of their nervous systems and observing the effects on their behaviour. This method has since been used in a variety of disciplines, but most prominently in medical and psychological research and neuroscience.

## What Does it Have to Do with Machine Learning?

In the context of machine learning, we can define ablation study as “a scientific examination of a machine learning system by removing its building blocks in order to gain insight on their effects on its overall performance”. Dataset features and model components are notable examples of these building blocks (hence we use their corresponding terms of feature ablation and model ablation), but any design choice or module of the system may be included in an ablation study.

## Experiments and Trials

We can think that an ablation study is an experiment that consists of several trials. For example, each model ablation trial involves training a model with one or more of its components (e.g. a layer) removed. Similarly, a feature ablation trial involves training a model using a different set of dataset features, and observing the outcomes.

The image above shows a sample feature ablation trial (up) and a sample layer ablation trial (down). For example, in the sample layer ablation trial, we remove the last hidden layer from the base model, train the resulting model, and observe its accuracy.

## Ablation Studies with Maggy

With Maggy, performing ablation studies of your machine learning or deep learning systems is a fairly simple task that consists of the following steps:

1. Creating an AblationStudy instance,
2. Specifying the components that you want to ablate by including them in your AblationStudy instance,
3. Defining a base model generator function and/or a dataset generator function,
4. Wrapping your TensorFlow/Keras code in a Python function (let’s call it training function) that receives two arguments (model_function and dataset_function), and
5. Launching your experiment with Maggy while specifying an ablation policy.

It’s as simple as that.

## What Changes Should I Make in my TensorFlow/Keras Code?

Not so much. You’ll see an example shortly, but the most important thing is:

• For model ablation, you need to define a function that returns a TF/Keras model, and use that in your code instead of defining the model in your training function. If you want to perform layer ablation, then you should provide a name argument while adding layers to your tf.keras.Sequential model, and include those names in your AblationStudy instance as well.

• For feature ablation:

• if you have your training dataset in the Feature Store (congratulations! :D) in form of tfrecord, you can directly include the features you want to ablate using their names and calling a dataset generator function in your training function. The dataset generator functions will be created under the hood by maggy for each feature ablation trial.
• alternatively, you can define your own dataset generator function and include it in your AblationStudy instance. A separate example notebook will be created to show you how you can do this.

Now let’s see how this actually works. Get your SparkSession by executing the following cell:

from hops import hdfs
from hops import featurestore
import maggy
Starting Spark application

IDYARN Application IDKindStateSpark UIDriver log
SparkSession available as 'spark'.


The next step is to create an AblationStudy instance. Here, the required arguments are 1) the name of your training dataset as it is in your project’s feature store, and 2) * the name of the label column.

You can also provide the version of your training dataset in the feature store, but the default version is 1.

# create an AblationStudy instance.

from maggy.ablation import AblationStudy

ablation_study = AblationStudy('titanic_train_dataset', training_dataset_version=1,
label_name='survived')

## Feature Ablation

We perform feature ablation by including features in our AblationStudy instance. Including a feature means that there will be a trial where the model will be trained without that feature. In other words, you include features in the ablation study so that they will be excluded from the training dataset.

We have the following features in our training dataset:

['age', 'fare', 'parch', 'pclass', 'sex', 'sibsp', 'survived']

You can include features using features.include() method of your AblationStudy instance, by passing the names of the features, either separately or as a list of strings:

#include features one by one

ablation_study.features.include('pclass')

# include a list of features

list_of_features = ['fare', 'sibsp']
ablation_study.features.include(list_of_features)

Let’s see what features we have in our ablation study, using features.list_all():

ablation_study.features.list_all()
pclass
sibsp
fare


So, we have the above features included in our ablation study. Based on the configuration we have so far, once we launch the ablation study experiment we will have 4 trials: - a trial with all the features, except for fare, - another trial with all the features except sibsp, - another one again with all the features, except for pclass, - and of course, the base trial that has all the features of the training dataset.

Now let’s see how we can perform model ablation with Maggy.

## Model Ablation

By model ablation we mean removing the components of the model and observing the resulting performance. Depending on which component of the model you want to ablate, we could have different types of model ablation, but one thing they all share in common is that we should have one base model in order to compare the other models with it. So we should define a base model generator function that returns a tf.keras.Model.

Maybe the simplest type of model ablation that can be performed on a sequential deep learning model is to remove some of its layers, so let’s just do that. We call this layer ablation. In Keras, when you are adding layers to your Sequential model, you can provide a name argument with a custom name. The Maggy ablator then uses these names to identify and remove the layers. Then, for each trial, the ablator returns a corresponding model generator function that differs from the base model generator in terms of its layers.

By the way, if you do not provide a name argument while adding a layer, that layer’s name will be prefixed by its layer class name, followed by an incremental number, e.g., dense_1.

In the following cell, we define a base model generator function that once executed will return a Sequential model:

# define the base model generator function

def base_model_generator():
import tensorflow as tf
model = tf.keras.Sequential()
# output layer
return model

After that, we should pass the base model generator function to our AblationStudy instance:

# set the base model generator

ablation_study.model.set_base_model_generator(base_model_generator)

### Single Layers

Adding layers to your ablation study is easy - just pass their names to model.layers.include(), or pass a list of strings of the names. Of course, these names should match the names you define in your base model generator function:

# include some single layers in the ablation study

ablation_study.model.layers.include('my_dense_two', 'my_dense_three', 'my_dense_four', 'my_dense_sigmoid')
# see the included single layers

ablation_study.model.layers.print_all()
Included single layers are:

my_dense_three
my_dense_sigmoid
my_dense_four
my_dense_two


### Layer Groups

For simple, not-so-deep models, it might make sense to add the layers to the model one-by-one. However, in many well-known neural network architectures, we have tens or hundreds of layers that sometimes come in blocks or modules, and are usually generated using constructs like for loops.

In Maggy, you can easily include such layers in your ablation study experiment, using model.layers.include_groups() method of your AblationStudy instance. You can either pass it a list of layers that should be regarded as a single layer group, or provide it with a prefix argument:

# add a layer group using a list

ablation_study.model.layers.include_groups(['my_dense_two', 'my_dense_four'])
# add a layer group using a prefix

ablation_study.model.layers.include_groups(prefix='my_dense')
ablation_study.model.layers.print_all_groups()
Included layer groups are:

---- All layers prefixed "my_dense"
--- Layer group ['my_dense_four', 'my_dense_two']


### Custom Models

Sometimes our models are defined in a way that ablating them takes much more than removing individual layers or layer groups. Complex models developed using Keras’ Functional API or with custom classes, such as VGG-16 or Inception-v3, are examples of such models, and they may include skip connections, branches, and arbitrary topologies.

You can still use Maggy for ablation studies of such complex models. To do that, you just have to - again - extract the model generation logic into a python callable, the same way you defined your base model generator, and include it in the study using model.add_custom_model_generator(). This method takes two arguments: - custom_model_generator: a python callable that returns a model. - model_identifier: a string to be used as an identifier for the custom model.

In the following example we change the simple model that we defined earlier as our base model generator, and include it in our study. Note that adding each custom model leads to the addition of a single trial to our study.

def my_custom_model():
model = tf.keras.Sequential()
# output layer
return model

ablation_study.model.add_custom_model_generator(my_custom_model, 'a custom model')

Just to recap, the ablator will generate one trial per each single layer and one trial per each layer group, and one trial for the custom model we defined above.

Now the only thing you need to do is to wrap your training code in a Python function. You can name this function whatever you wish, but we will refer to it as the training or wrapper function. The model_function and dataset_function used in the code are generated by the ablator per each trial, and you should call them in your code. This is your everyday TensorFlow/Keras code:

# wrap your code in a Python function

from maggy import experiment

def training_fn(dataset_function, model_function):
import tensorflow as tf
epochs = 5
batch_size = 10
tf_dataset = dataset_function(epochs, batch_size)
model = model_function()
loss='binary_crossentropy',
metrics=['accuracy'])

history = model.fit(tf_dataset, epochs=5, steps_per_epoch=30, verbose=0)
return float(history.history['accuracy'][-1])

And, lagom! Lagom is a Swedish word that means “just the right amount”, and that is how Maggy uses your resources to for parallel trials. You should provide lagom with an ablator. So far, we have implemented the most natural ablator of them all: LOCO, which stands for “Leave One Component Out”. This ablator will generate one trial per each component included in the ablation study. However, Maggy’s developer API allows you to define your own ablator, in case you want to get creative.

You can also set a name for your experiment so that you can keep history or track its progress in Hopsworks.

Let’s lagom our experiment!

# Create a config for lagom
from maggy.experiment_config import AblationConfig

config = AblationConfig(name="Titanic-LOCO", ablation_study=ablation_study, ablator="loco", description="", hb_interval=1)
# launch the experiment

result = experiment.lagom(train_fn=training_fn, config=config)
HBox(children=(FloatProgress(value=0.0, description='Maggy experiment', max=11.0, style=ProgressStyle(descript…

Started Maggy Experiment: Titanic-LOCO, application_1586184410703_0006, run 1

------ LOCO Results ------
BEST Config Excludes {"ablated_feature": "None", "ablated_layer": "['my_dense_four', 'my_dense_two']"} -- metric 0.7233333587646484
WORST Config Excludes {"ablated_feature": "None", "ablated_layer": "Layers prefixed my_dense"} -- metric 0.3866666555404663
AVERAGE metric -- 0.6309090906923468
Total Job Time 0 hours, 2 minutes, 36 seconds

Finished Experiment