President Barack Obama speaks in the White House briefing room in Washington on May 6. (Carolyn Kaster/Associated Press)

In his Dec. 19 op-ed, “A legacy that can’t be repealed,” Robert J. Samuelson credited President Obama with preventing the Great Recession from becoming a great depression. I voted for Mr. Obama in 2008 and 2012. I agree with Mr. Samuelson that the president probably did prevent the Great Recession from becoming much worse. Nevertheless, economists cannot prove their theories the way chemists and physicists can with controlled, repeatable experiments. We cannot go back in time, choose a different policy and measure different results.

Median income adjusted for inflation has declined since Mr. Obama was inaugurated. This may be why Donald Trump was elected. It is why Republicans have been able to prolong the delusion that they are better managers of the economy.

John Engelman, Wilmington, Del.