  It has been observed that real-world social networks often exhibit
stratification along economic or other lines, with consequences for class
mobility and access to opportunities. With the rise in human interaction data
and extensive use of online social networks, the structure of social networks
(representing connections between individuals) can be used for measuring
stratification. However, although stratification has been studied extensively
in the social sciences, there is no single, generally applicable metric for
measuring the level of stratification in a network.
  In this work, we first propose the novel Stratification Assortativity (StA)
metric, which measures the extent to which a network is stratified into
different tiers. Then, we use the \texttt{StA} metric to perform an in-depth
analysis of the stratification of five co-authorship networks. We examine the
evolution of these networks over 50 years and show that these fields
demonstrate an increasing level of stratification over time, and,
correspondingly, the trajectory of a researcher's career is increasingly
correlated with her entry point into the network.
