Multilabel Text Classification using TensorFlow

Question

The text data is organized as vector with 20,000 elements, like [2, 1, 0, 0, 5, ...., 0]. i-th element indicates the frequency of the i-th word in a text.

The ground truth label data is also represented as vector with 4,000 elements, like [0, 0, 1, 0, 1, ...., 0]. i-th element indicates whether the i-th label is a positive label for a text. The number of labels for a text differs depending on texts.

I have a code for single-label text classification.

How can I edit the following code for multilabel text classification?

Especially, I would like to know following points.

• How to compute accuracy using TensorFlow.
• How to set a threshold which judges whether a label is positive or negative. For instance, if the output is [0.80, 0.43, 0.21, 0.01, 0.32] and the ground truth is [1, 1, 0, 0, 1], the labels with scores over 0.25 should be judged as positive.

Thank you.

import tensorflow as tf

# hidden Layer
class HiddenLayer(object):
    def __init__(self, input, n_in, n_out):
        self.input = input

        w_h = tf.Variable(tf.random_normal([n_in, n_out],mean = 0.0,stddev = 0.05))
        b_h = tf.Variable(tf.zeros([n_out]))

        self.w = w_h
        self.b = b_h
        self.params = [self.w, self.b]

    def output(self):
        linarg = tf.matmul(self.input, self.w) + self.b
        self.output = tf.nn.relu(linarg)

        return self.output

# output Layer
class OutputLayer(object):
    def __init__(self, input, n_in, n_out):
        self.input = input

        w_o = tf.Variable(tf.random_normal([n_in, n_out], mean = 0.0, stddev = 0.05))
        b_o = tf.Variable(tf.zeros([n_out]))

        self.w = w_o
        self.b = b_o
        self.params = [self.w, self.b]

    def output(self):
        linarg = tf.matmul(self.input, self.w) + self.b
        self.output = tf.nn.relu(linarg)

        return self.output

# model
def model():
    h_layer = HiddenLayer(input = x, n_in = 20000, n_out = 1000)
    o_layer = OutputLayer(input = h_layer.output(), n_in = 1000, n_out = 4000)

    # loss function
    out = o_layer.output()
    cross_entropy = -tf.reduce_sum(y_*tf.log(out + 1e-9), name='xentropy')    

    # regularization
    l2 = (tf.nn.l2_loss(h_layer.w) + tf.nn.l2_loss(o_layer.w))
    lambda_2 = 0.01

    # compute loss
    loss = cross_entropy + lambda_2 * l2

    # compute accuracy for single label classification task
    correct_pred = tf.equal(tf.argmax(out, 1), tf.argmax(y, 1))
    accuracy = tf.reduce_mean(tf.cast(correct_pred, "float"))

    return loss, accuracy

Answer 1

You have to use variations of cross entropy function in other to support multilabel classification. In case you have less than one thousand of ouputs you should use sigmoid_cross_entropy_with_logits, in your case that you have 4000 outputs you may consider candidate sampling as it is faster than the previous.

How to compute accuracy using TensorFlow.

This depends on your problem and what you want to achieve. If you don't want to miss any object in an image then if the classifier get all right but one, then you should consider the whole image an error. You can also consider that an object missed or missclassiffied is an error. The latter I think it supported by sigmoid_cross_entropy_with_logits.

How to set a threshold which judges whether a label is positive or negative. For instance, if the output is [0.80, 0.43, 0.21, 0.01, 0.32] and the ground truth is [1, 1, 0, 0, 1], the labels with scores over 0.25 should be judged as positive.

Threshold is one way to go, you have to decided which one. But that is some kind of hack, not real multilable classification. For that you need the previous functions I said before.

Answer 2

Change relu to sigmoid of output layer. Modify cross entropy loss to explicit mathematical formula of sigmoid cross entropy loss (explicit loss was working in my case/version of tensorflow )

import tensorflow as tf

# hidden Layer
class HiddenLayer(object):
    def __init__(self, input, n_in, n_out):
        self.input = input

        w_h = tf.Variable(tf.random_normal([n_in, n_out],mean = 0.0,stddev = 0.05))
        b_h = tf.Variable(tf.zeros([n_out]))

        self.w = w_h
        self.b = b_h
        self.params = [self.w, self.b]

    def output(self):
        linarg = tf.matmul(self.input, self.w) + self.b
        self.output = tf.nn.relu(linarg)

        return self.output

# output Layer
class OutputLayer(object):
    def __init__(self, input, n_in, n_out):
        self.input = input

        w_o = tf.Variable(tf.random_normal([n_in, n_out], mean = 0.0, stddev = 0.05))
        b_o = tf.Variable(tf.zeros([n_out]))

        self.w = w_o
        self.b = b_o
        self.params = [self.w, self.b]

    def output(self):
        linarg = tf.matmul(self.input, self.w) + self.b
        #changed relu to sigmoid
        self.output = tf.nn.sigmoid(linarg)

        return self.output

# model
def model():
    h_layer = HiddenLayer(input = x, n_in = 20000, n_out = 1000)
    o_layer = OutputLayer(input = h_layer.output(), n_in = 1000, n_out = 4000)

    # loss function
    out = o_layer.output()
    # modified cross entropy to explicit mathematical formula of sigmoid cross entropy loss
    cross_entropy = -tf.reduce_sum( (  (y_*tf.log(out + 1e-9)) + ((1-y_) * tf.log(1 - out + 1e-9)) )  , name='xentropy' )    

    # regularization
    l2 = (tf.nn.l2_loss(h_layer.w) + tf.nn.l2_loss(o_layer.w))
    lambda_2 = 0.01

    # compute loss
    loss = cross_entropy + lambda_2 * l2

    # compute accuracy for single label classification task
    correct_pred = tf.equal(tf.argmax(out, 1), tf.argmax(y, 1))
    accuracy = tf.reduce_mean(tf.cast(correct_pred, "float"))

    return loss, accuracy