Incorrect number of Pull Requests in table pull_request_history

[Inventitech]

Using the query

SELECT action, COUNT(*) as freq, YEAR(created_at) as pr_yr, MONTH(created_at) as pr_mt
FROM
  [ghtorrent-bq:ght_2017_09_01.pull_request_history] 
GROUP BY pr_yr, pr_mt, action

I get a monthly digest of opened and merged PRs events on GitHub. Unfortunately, plotting this does not yield a very plausible graph:

I was able to figure out when things seem to have gone wrong for the number of merged pullrequests, to the first month of 2014:

Later, the sharp drop in mid-2016 also seems questionable.

I hope this helps you debug the issue.

(Possibly related to #19.)