Does bias in the convolutional layer really make a difference to the test accuracy?

问题

I understand that bias are required in small networks, to shift the activation function. But in the case of Deep network that has multiple layers of CNN, pooling, dropout and other non -linear activations, is Bias really making a difference? The convolutional filter is learning local features and for a given conv output channel same bias is used.

This is not a dupe of this link. The above link only explains role of bias in small neural network and does not attempt to explain role of bias in deep-networks containing multiple CNN layers, drop-outs, pooling and non-linear activation functions.

I ran a simple experiment and the results indicated that removing bias from conv layer made no difference in final test accuracy. There are two models trained and the test-accuracy is almost same (slightly better in one without bias.)

model_with_bias,
model_without_bias( bias not added in conv layer)

Are they being used only for historical reasons?

If using bias provides no gain in accuracy, shouldn't we omit them? Less parameters to learn.

I would be thankful if someone who have deeper knowledge than me, could explain the significance(if- any) of these bias in deep networks.

Here is the complete code and the experiment result bias-VS-no_bias experiment

batch_size = 16
patch_size = 5
depth = 16
num_hidden = 64

graph = tf.Graph()

with graph.as_default():

  # Input data.
  tf_train_dataset = tf.placeholder(
    tf.float32, shape=(batch_size, image_size, image_size, num_channels))
  tf_train_labels = tf.placeholder(tf.float32, shape=(batch_size, num_labels))
  tf_valid_dataset = tf.constant(valid_dataset)
  tf_test_dataset = tf.constant(test_dataset)

  # Variables.
  layer1_weights = tf.Variable(tf.truncated_normal(
      [patch_size, patch_size, num_channels, depth], stddev=0.1))
  layer1_biases = tf.Variable(tf.zeros([depth]))
  layer2_weights = tf.Variable(tf.truncated_normal(
      [patch_size, patch_size, depth, depth], stddev=0.1))
  layer2_biases = tf.Variable(tf.constant(1.0, shape=[depth]))
  layer3_weights = tf.Variable(tf.truncated_normal(
      [image_size // 4 * image_size // 4 * depth, num_hidden], stddev=0.1))
  layer3_biases = tf.Variable(tf.constant(1.0, shape=[num_hidden]))
  layer4_weights = tf.Variable(tf.truncated_normal(
      [num_hidden, num_labels], stddev=0.1))
  layer4_biases = tf.Variable(tf.constant(1.0, shape=[num_labels]))

  # define a Model with bias .
  def model_with_bias(data):
    conv = tf.nn.conv2d(data, layer1_weights, [1, 2, 2, 1], padding='SAME')
    hidden = tf.nn.relu(conv + layer1_biases)
    conv = tf.nn.conv2d(hidden, layer2_weights, [1, 2, 2, 1], padding='SAME')
    hidden = tf.nn.relu(conv + layer2_biases)
    shape = hidden.get_shape().as_list()
    reshape = tf.reshape(hidden, [shape[0], shape[1] * shape[2] * shape[3]])
    hidden = tf.nn.relu(tf.matmul(reshape, layer3_weights) + layer3_biases)
    return tf.matmul(hidden, layer4_weights) + layer4_biases

  # define a Model without bias added in the convolutional layer.
  def model_without_bias(data):
    conv = tf.nn.conv2d(data, layer1_weights, [1, 2, 2, 1], padding='SAME')
    hidden = tf.nn.relu(conv ) # layer1_ bias is not added 
    conv = tf.nn.conv2d(hidden, layer2_weights, [1, 2, 2, 1], padding='SAME')
    hidden = tf.nn.relu(conv) # + layer2_biases)
    shape = hidden.get_shape().as_list()
    reshape = tf.reshape(hidden, [shape[0], shape[1] * shape[2] * shape[3]])
    # bias are added only in Fully connected layer(layer 3 and layer 4)
    hidden = tf.nn.relu(tf.matmul(reshape, layer3_weights) + layer3_biases)
    return tf.matmul(hidden, layer4_weights) + layer4_biases

  # Training computation.
  logits_with_bias = model_with_bias(tf_train_dataset)
  loss_with_bias = tf.reduce_mean(
    tf.nn.softmax_cross_entropy_with_logits(labels=tf_train_labels, logits=logits_with_bias))

  logits_without_bias = model_without_bias(tf_train_dataset)
  loss_without_bias = tf.reduce_mean(
    tf.nn.softmax_cross_entropy_with_logits(labels=tf_train_labels, logits=logits_without_bias))

  # Optimizer.
  optimizer_with_bias = tf.train.GradientDescentOptimizer(0.05).minimize(loss_with_bias)
  optimizer_without_bias = tf.train.GradientDescentOptimizer(0.05).minimize(loss_without_bias)

  # Predictions for the training, validation, and test data.
  train_prediction_with_bias = tf.nn.softmax(logits_with_bias)
  valid_prediction_with_bias = tf.nn.softmax(model_with_bias(tf_valid_dataset))
  test_prediction_with_bias = tf.nn.softmax(model_with_bias(tf_test_dataset))

  # Predictions for without
  train_prediction_without_bias = tf.nn.softmax(logits_without_bias)
  valid_prediction_without_bias = tf.nn.softmax(model_without_bias(tf_valid_dataset))
  test_prediction_without_bias = tf.nn.softmax(model_without_bias(tf_test_dataset))

num_steps = 1001

with tf.Session(graph=graph) as session:
  tf.global_variables_initializer().run()
  print('Initialized')
  for step in range(num_steps):
    offset = (step * batch_size) % (train_labels.shape[0] - batch_size)
    batch_data = train_dataset[offset:(offset + batch_size), :, :, :]
    batch_labels = train_labels[offset:(offset + batch_size), :]
    feed_dict = {tf_train_dataset : batch_data, tf_train_labels : batch_labels}
    session.run(optimizer_with_bias, feed_dict=feed_dict)
    session.run(optimizer_without_bias, feed_dict = feed_dict)
  print('Test accuracy(with bias): %.1f%%' % accuracy(test_prediction_with_bias.eval(), test_labels))
  print('Test accuracy(without bias): %.1f%%' % accuracy(test_prediction_without_bias.eval(), test_labels))

Output:

Initialized

Test accuracy(with bias): 90.5%

Test accuracy(without bias): 90.6%

回答1:

Biases are tuned alongside weights by learning algorithms such as gradient descent. biases differ from weights is that they are independent of the output from previous layers. Conceptually bias is caused by input from a neuron with a fixed activation of 1, and so is updated by subtracting the just the product of the delta value and learning rate.

In a large model, removing the bias inputs makes very little difference because each node can make a bias node out of the average activation of all of its inputs, which by the law of large numbers will be roughly normal. At the first layer, the ability for this to happens depends on your input distribution. For MNIST for example, the input's average activation is roughly constant. On a small network, of course you need a bias input, but on a large network, removing it makes almost no difference.