Moments after Steve Bannon’s departure from the White House earlier this month, news outlets were quick to report on the high number of top advisers with whom President Trump has parted ways. The number of departures may feel like a lot, but are the shake-ups that unusual? In one sense, perhaps not: Historically, turnover of presidential advisers has been increasing since the 1970s. But in another sense, yes: The speed and timing of the exodus from the Trump administration is far exceeding even that increased pace.

One reason it’s not immediately obvious whether Trump’s retention rates are unusual is that there’s no consensus over who counts as a top adviser. I wanted a more definite count than the varying numbers in news reports about staffers who have left the White House, so I defined “top adviser” as administration officials who are registered as “assistant to the president.” This is the highest formal rank of executive branch official. Since 1995, presidents have been required to submit a report to Congress each year on all White House staff, and it just so happens that they designate whether someone has this formal title.

Trump released his first report on June 30, 2017, and it included 26 people with the title “assistant to the president.” Note that this group excludes someone like Sebastian Gorka, a controversial deputy assistant to the president, who left the White House last week. In just the eight weeks since the report was released, four have left or are reportedly expected to leave (Reince Priebus, Steve Bannon, Kathleen McFarland, and Sean Spicer) — that’s an 84.6 percent retention rate in a little under two months. This number doesn’t include at least four assistants to the president who had either already left the White House before the report was released (Katie Walsh, Mike Dubke and Michael Flynn), or were both hired and left after the report was released (Anthony Scaramucci).

We can calculate Obama’s annual retention rates the same way. Between his first and second annual report, he retained 86.4 percent of staffers with the title “assistant to the president.” At 84.6 percent, Trump’s two-month number is already slightly below Obama’s rate over a full 12 months — even though Trump has had 10 fewer months to lose staffers.

Both Obama’s and Trump’s numbers are part of a larger decline in adviser retention by U.S. presidents that was written about in a paper published in 2002 in The Journal of Politics. Political scientists Matthew Dickinson and Kathryn Dunn Tenpas examined retention rates for all top White House staff and Cabinet members who were listed in the U.S. Government Manual, an annual government record of all agencies, personnel and their roles, between 1929 and 1997, the first year of Bill Clinton’s second term. They found that before 1971, the average annual retention rate among these government employees was 78 percent. Post-1971, the average annual rate dropped to 68 percent.

Dickinson and Tenpas argue that this decline is the result of a change in how campaigns are run. A series of reforms in the late 1960s and early 1970s pushed candidates, rather than political parties, to manage their own presidential campaigns. This meant candidates hired more people with experience in campaigning — or marketing, or selling a personality — than people with governing experience. Yet presidents continued to hire campaign aides to be their advisers when they assumed office. Dickinson and Tenpas theorize that that has led to an increase in inexperienced top presidential advisers, who depart as the poor fit becomes apparent.

Importantly, the numbers in Dickinson and Tenpas’s paper can’t be directly compared to the retention rates of Trump’s and Obama’s above, for two reasons. First, as noted above, the researchers cast a wider net for a president’s top officials than I did, considering not only the “assistant to the president” designation but also other assistants and Cabinet members. Secondly, their analysis included retention rates between presidential administrations, which, obviously, are extremely low (if not zero). That has the effect of deflating the average across the board.

But, we can still compare trends. Here are the retention rates I found for Obama’s top advisers for every year of his administration outside of the first, based on how his top staffers changed from one annual report to the next:

From 2009 to 2010: 86.4 percent

2010-11: 50.0 percent

2011-12: 76.2 percent

2012-13: 59.1 percent

2013-14: 45.5 percent

2014-15: 73.9 percent

2015-16: 86.4 percent

Notice that in the Obama numbers we see a dip in retention between July 2010 through July 2011, which overlaps with the Democrats’ “shellacking” in the 2010 midterms. Obama’s shedding of staffers a couple years into his term is consistent with the Dickinson and Tenpas theory that we see staff retention drop as the poor skill fit between campaign staff and governing staff becomes apparent. We also see a dip as Obama approaches re-election in 2012 and resets his administration after he’s elected, as well as a return to stability near the end of both terms, all of which is also consistent with their argument.

So what might this mean for Trump? There are two possibilities. Perhaps he’s learning faster than Obama and others and replacing his advisers in the first few months of his administration, rather than waiting more than a year. Alternatively, if these replacements just represent a new baseline, we could be in for more turbulence in coming years. One test will be to see how many advisers make it to next year’s report: if we see stability, Trump may be a fast learner. If not, then Trump may have set a new standard for instability.