Editor's note: Julian E. Zelizer is a professor of history and public affairs at Princeton University. He is the author of "Jimmy Carter," published by Times Books, and editor of a book assessing former President George W. Bush's administration, published by Princeton University Press.
Princeton, New Jersey (CNN) -- Since the late 1940s, it has been an American custom for pollsters and publications to release a ranking of U.S. presidents.
Usually based on a survey of historians and journalists or of the public, the ranking informs readers about who the "best" and "worst" presidents are. In an age when we are constantly desperate to craft Top 10 lists for every part of our lives, this approach to political history is appealing.
But rankings don't tell us much about presidential history. The rankings are weak mechanisms for evaluating what has taken place in the White House.
According to the Siena Research Poll's survey of presidential scholars in 2010, Franklin Roosevelt, Theodore Roosevelt, Abraham Lincoln, George Washington and Thomas Jefferson were the top presidents.
In 2009, C-SPAN declared that Lincoln, Washington, Harry Truman and the Roosevelts were at the top of their list -- and Rutherford B. Hayes, Herbert Hoover, John Tyler, Millard Fillmore and George W. Bush were at the bottom.
Most recently, Gallup found that the public ranks Ronald Reagan, Lincoln, Bill Clinton, John Kennedy and Washington as the greatest presidents. Franklin Roosevelt and Barack Obama came in right under them, both ahead of Theodore Roosevelt and Thomas Jefferson.
There are many flaws with the system. The first is that presidential reputations vary over time. Any ranking simply captures how people view the presidents at a given moment. It is not a definitive measure. Although there are a few presidents who constantly hover at the top, such as Abraham Lincoln and Franklin Roosevelt, most experience significant fluctuations over time.
Take the case of Dwight Eisenhower, who was president from 1953 to 1961. Initially, many observers believed that Eisenhower was a weak leader, someone who exerted little influence over his Cabinet and who essentially allowed his advisers to call the shots. But research in the presidential archives (where the White House deposits its records for scholars to examine) later revealed that this perception was totally incorrect.
Political scientist Fred Greenstein and historian Robert Griffith both found that behind the scenes, Eisenhower maintained a strong hold on decision-making. In what he called the "hidden hand presidency," Greenstein presented Eisenhower as an effective decision maker who maintained control over his Cabinet.
Early interpretations of John F. Kennedy from the New Left were not kind. They asserted that Kennedy had attempted to protect corporate interests and ward off radical changes that were being demanded by civil rights activists. That analysis changed as well. More sympathetic accounts were subsequently published that argued that Kennedy was truly committed to the liberal agenda and that he accomplished a great deal through executive power and his leadership skills, even though he often found himself frustrated by conservative opponents on Capitol Hill.
Nowhere was the shift more dramatic than with Reagan. Soon after he finished his term, many historical accounts depicted Reagan as a shallow Hollywood actor who parroted what his advisers told him and who didn't have a firm grasp of policy issues. But the historical research quickly challenged this assertion as well.
New books revealed that Reagan had immersed himself in the ideas of conservatism since the 1950s. When he hosted a radio show in the late 1970s, he finely crafted every word of his broadcasts to develop the ideas that shaped his presidency.
While in the White House, Reagan governed with a strong hand in foreign policy and tightly managed the negotiations with Mikhail Gorbachev. Although many historians are still highly critical of his presidency, few doubt that he was far more politically skilled and intellectually capable than some initially said.
Presidential rankings also change depending on the context in which presidents are being evaluated. When Harry Truman finished his presidency in 1953, he was roundly criticized as an ineffective and bumbling leader. Even many liberals thought that he paled in comparison to Franklin Roosevelt. Yet over time, as the Cold War unfolded and came to an end, historians took another look.
By the late 1980s, historians were writing about Truman as the architect of national security policies enacted between 1947 and 1949 that served as the foundation of the nation's battle against communism.
Some experts are already starting to review President George W. Bush's national security record in a different light. Because President Obama has continued many of Bush's homeland security and interrogations policies and because of the recent outbreak of social protest in the Middle East, there have been discussions about whether Bush's policies -- including his push for democracy in the region -- were more on target and necessary than many had suspected.
Moreover, it is unclear what we are measuring when we talk about presidential greatness. The rankings assume some kind of common criteria. Yet success in politics means different things to different people. For some scholars, presidential success means a huge legislative record. Any president who remakes the legislative landscape, such FDR or Lyndon Johnson, and some would say Barack Obama, deserves to be at the top of a list.
But others disagree. They say that legislative productivity can produce a lot of bad legislation. Doing a lot doesn't mean doing great things. Moreover, this measure ignores presidents who don't have as much success on Capitol Hill but have other types of impact, such Reagan's ability to reshape public debate by pushing it toward the right.
Most presidencies are too complex to simply be labeled good or bad. For many observers, Johnson was a significant success. If we measure the amount of domestic legislation that he pushed through Congress and the long-term impact that legislation had, Johnson probably falls under the category of great. Just the passage of the Civil Rights Act of 1964 and the Voting Rights Act of 1965 should be enough to place a president in the hall of fame. But once you bring the Vietnam War into the mix, the picture looks different.
Historians and journalists are often influenced by their own political biases when trying to figure out who is "bad" or "good." So too is the public. Politics doesn't stop at the rankings edge.
In the recent Gallup Poll, Republicans named Reagan as the greatest president while Democrats picked Clinton. And there might be a bias toward more recent presidents since survey respondents are likely to value a Reagan or a Clinton more highly than some of the earlier presidents simply because many of those responding to the poll lived through their terms in office.
These are just a few of the problems with presidential rankings. While they might be fun to read and make for a good headline, they aren't a great way to study history.
On Presidents Day, we would do well to take a more serious look back at the complexity of each man who inhabited the White House, focusing less on whether a president should be considered No. 1 or No. 15 like our favorite sports team and instead delving deeper into the events and decisions that shaped their time in office.
The opinions expressed in this commentary are solely those of Julian E. Zelizer