Evolution of the Network:

Since every podcast episode comes with a release date, we can track the changes in measures of the network over time. While there are a handful of podcasts in the network that were releasing episodes as early as 2005, podcasting didn't really take off until 2010. Therefore, all the evolution plots below start at the beginning of 2010. From 2010 to 2015 we see significant growth and rapid changes in centrality scores. From 2015 onwards, however, there is a marked increase in stability and a converging of values.

Global Measures:

The first two rows show the growth of the number of people and podcasts, respectively, in the network. They both have smooth growth rates, and the curve showing the number of people is reasonably well fit by an exponential. If we extrapolate the fit exponential equation, we predict that there will be over 22,000 people by the year 2020. At the start of 2015, when the stability of network measures suddenly increases, there is no significant jump in either the number of people nor the number of podcasts, which are just below 4000 and 80, respectively. Since then, the number of people has tripled and the number of podcasts has doubled.

The density of the network, which compares the number of links to the set of all possible links, is steadily decreasing over time. This implies that the rate at which people are being added to the network is faster than the rate at which existing nodes (people already in the network) are adding new links.

The average shortest path between all possible pairs of nodes grew to just over 4 by 2011 and has stayed there relatively stably since then. This implies that the diameter of the network is not changing too much and that large clusters and cliques are not having a significant impact on the structure of the network.

The transitivity and the average clustering coefficient, the last two rows in the plot, are two ways of measuring the global clustering properties of the network. Both have very low values due to the fact that guest cannot directly link to other guests, only to hosts. Therefore, these clustering measures are determined by the frequency of podcast hosts being guests on other podcasts. Both of these measures also show the same decreased variability after 2015 discussed above.

Pagerank Evolution:

Here we are plotting the chaning Pageranks for the people with the current top ten Pageranks. We see the high variability before 2015, then much smoother curves after 2015. Also, after 2015 all the Pageranks are decreasing, as new podcasts with new hosts enter the network and dilute the pagerank scores. Joe Rogan clearly has by far the largest Pagerank. Duncan Trussell, who is not nearly as central by other measures, has such a high Pagerank due to the fact that he has been on The Joe Rogan Experience 45 times for a total time of almost 5 days, and Joe Rogan has been on The Duncan Trussell Family Hour 15 times for a total time of almost a day and a half. These very strong mutual links boost Duncan Trussell's score significantly.

Authority Score Evolution:

The authority scores evolutions are smoother than those of Pagerank, but show the same decrease in variability after 2015. Chris Hardwick, host of ID10T with Chris Hardwick, had by far the highest authority score from 2012 to early 2014, as consistenly had celebrities and comedians as guests more than twice a week. This time period was relatively early in the podcast network, when it was relatively easier to "shine" in the podcast world, but by 2015 there were enough podcasts to dilute Chris Hardwick's authority score.

In this plot there are three pairs of people whose curves are almost completely overlapping, where all three pairs are co-hosts of a particular podcast. Cameron Esposito and Rhea Butcher are co-hosts of Put Your Hands Together with Cam and Rhea, so their authority scores are overlapping until about 2018, due to Cameron Esposito starting her own podcast, QUEERY with Cameron Esposito, in mid-2017. Similarly, Jesse Thorn and Jordan Morris co-host Jordan, Jesse GO! and have overlapping authority scores until mid-2016 when Jesse Thorn started his own podcast, Bullseye with Jesse Thorn. Ryan Sickler and Jay Larson, co-hosts of The CrabFeast with Ryan Sickler and Jay Larson, have exactly overlapping authority scores the whole time.

Hub Score Evolution:

The most outstanding feature in this figure is the spike in the hub scores of Moshe Kasher and Eddie Pepitone in late 2010 that lasts until 2013. Since we are only showing the evolution of the hub scores of the *current* top ten, there may be others in this time period that had similar scores but are no longer in the top ten. From 2013 to 2015 there is signifcant variability that dies down by 2015.

Closeness Centrality Evolution:

The closeness centrality scores are the only ones that are, for the most part, monotonically increasing. The growth rates of the closeness centrality decreases after 2015, but the variability seems rather constant. Also, unlike the other measures, they continue to increase post-2015. The closeness centrality is a measure that is not *diluted* by the addition of new people into the network.

Chris Hardwick has the highest closeness centrality from the beginning until early 2017 when he is surpassed by Joe Rogan, and then surpassed again in early 2018 by Pete Holmes.