Washington (CNN) You may have read about the new study from political scientists that ranks President Donald Trump as the worst president ever. There's nothing inherently wrong with the study, and the political scientists who worked on it are well-respected. Most Americans, though, don't rank Trump as the worst president ever , even if most disapprove of the job Trump is doing .

Indeed, there is good reason to be cautious in using this study of political scientists to determine what Trump's place in history is at this point or what it will ultimately be. Why?

1. The survey respondents are not Trump friendly

The majority of respondents (57%) self-identified as Democrats compared to just 13% who were Republicans. That 44-percentage-point split is about 35 to 40 points wider than what is found in the general public, according to polls from Gallup . Given that Trump's approval rating nationally among Democrats is in the single digits , it shouldn't be too surprising that his ranking among a very Democratic-leaning sample of political scientists is quite low.

Interestingly, the survey also found that Republican political scientists were also not fans of Trump. The 21 of them who took part in the survey (a small sample size to be sure) ranked Trump an average of 40th out of 44 presidents. In other words, the survey respondents didn't just hate Trump because he was a Republican.

These Republican political scientists probably aren't your normal Republicans, however. Although there is no reliable polling data that I'm aware of on Republican political scientists and their vote choice in 2016, I decided to take a look at how the precincts around some college campuses voted in the 2016 Republican presidential primary. I did so to get an idea of how these political scientists might have voted. Specifically, I looked at the precincts around Harvard University ( Cambridge ), the University of Alabama ( Tuscaloosa ), University of Michigan ( Ann Arbor ), University of North Carolina ( Chapel Hill ), University of Texas ( Austin ) and the University of Wisconsin ( Madison ). I chose these campuses because they are in different parts of the country. All of them, with the exception of Harvard, are also flagship state universities in states where Trump won the general election.

Trump finished first in the primary in none of these areas. In fact, he never finished above 30% of the vote. Interestingly, Ted Cruz didn't win any of these areas either. Instead, John Kasich and Marco Rubio won them, despite the fact that neither of them won or even came close to winning any of the states these contests were held in.

2. These surveys are capturing a moment in time among a specific subset

Keep in mind, those polled were a very specific segment of the population. They are "current and recent members of the Presidents & Executive Politics Section of the American Political Science Association." The results could have been different if they polled historians, for example.

Just last year, C-SPAN surveyed a group of mostly historians . Now, Trump wasn't asked about in that C-SPAN sample. Still, there were some differences between the survey of political scientists and historians. C-SPAN found James Polk ranked 14th instead of 20th as the political scientists did. They put John Adams at 19th instead of 14th. Both of these differences occurred, even though neither Adams nor Polk has been president in over 150 years.

Even among political scientists, these surveys don't necessarily stay consistent. Ulysses Grant, who was president in the 1870s, rose from 28th in a 2014 survey of political scientists to 21st in this one. Andrew Jackson, who was president in the 1830s, fell from ninth to 15th.

What changed? There could have been different political scientists taking the survey. Some political scientists may have also changed their mind. Some zeitgeist factors could be in play: Grant was the recent focus of a widely acclaimed biography by "Alexander Hamilton" author Ron Chernow while Jackson's name has been on the chopping block for state-level Democratic fundraisers and possibly the 20-dollar bill. (Jackson, by the way, is a favorite of Trump's, so that could also be attention that hasn't helped him with this crowd.) In other words, popular opinion may have changed on presidents in a very short time for presidents who haven't been in office for over 100 years.

3. Modern presidents are even more difficult to judge

The single-digit movement among presidents who served more than 100 years ago is nothing compared to the last two presidents who have been out of office for more than five years. During his presidency, a 2005 Wall Street Journal poll of professors gave George W. Bush a ranking of 19th. A Siena College survey of presidential scholars just after his presidency in 2010 put Bush at 39th. The 2018 political scientist survey placed Bush at 30th.

Likewise, Bill Clinton has seen major movement in his ratings. A Wall Street Journal poll taken during the Clinton presidency in 2000 put him at 24th. He rose to eighth in the aforementioned 2014 political scientist survey. He dropped to 13th in the most recent one.

Now, each set of judges may have had good reasons to vote the way they did, but the fact that the ratings changed frequently and by a lot gives you an idea of the fragility of them.

The Trump administration still has years to go. Something could occur that ends up changing the course of history. Trump could end up being a disaster in most people's minds by the end of it. He could end up surprising some folks.

The bottom line is that we don't know what's going to happen. Just don't be surprised if you turn around in a few years and find that Trump's position in the presidential rankings has changed.