What is "valency", with regards to machine learning?

Question

This term came up a few times in the Tensorflow Dev Summit, and it shows up in the Tensorflow Extended documentation, but without any sort of definition. After a fair amount of googling, I don't see reference to it in any Statistics-related setting. Searching the Tensorflow repositories produces a few hits, but they're similarly unhelpful. The term does seem to be used in Chemistry, Psychology, and Linguistics, but those definitions appear to be unrelated.

Answer 1

Per the 2017 TFX paper http://stevenwhang.com/tfx_paper.pdf, TFX can calculate a number of stats on a dataset, including:

"The expected valency of the feature in each example, i.e., minimum and maximum number of values."

We can also look at the code that calculates valency in TFX. From what I can tell, it's designed to run on a feature that is an array, and counts the minimum and maximum number of values within that array for that feature:

# Extract the valency information of the feature.
valency = ''
if feature.HasField('value_count'):
  if (feature.value_count.min == feature.value_count.max and
      feature.value_count.min == 1):
    valency = 'single'
  else:
    min_value_count = ('[%d' % feature.value_count.min
                       if feature.value_count.HasField('min') else '[0')
    max_value_count = ('%d]' % feature.value_count.max
                       if feature.value_count.HasField('max') else 'inf)')
    valency = min_value_count + ',' + max_value_count

from: https://github.com/tensorflow/data-validation/blob/master/tensorflow_data_validation/utils/display_util.py#L68