Donald Trump’s favorable rating is now roughly equal to Hillary Clinton’s favorable rating. To the Trump skeptic, the result might be viewed as a short-lived post-convention bounce for the Republican nominee. To the Trump fan, it might be viewed as a sign of things to come as the summer turns to fall. History suggests that it really could be either: Sometimes how voters view a candidate immediately after the conventions sticks — sometimes it doesn’t. (That’s something to keep in mind as we watch Clinton’s numbers now that her convention has wrapped up.)

The post-convention favorability polls have been a hair more predictive than the pre-convention polls, but I wouldn’t make too much of the difference — a number of candidates who had great conventions ended up finding popularity fleeting afterward. You can see this in the table below, which shows the net favorability of both major party nominees in the final CBS News polls of every campaign since 1980, compared to their net favorability in the CBS polls taken before and after both conventions:

NET FAVORABILITY CHANGE YEAR CANDIDATE BEFORE CONV. AFTER CONV. FINAL DURING CONV. AFTER CONV. 1980 Carter -25 -1 -8 +24 -7 1980 Reagan +12 0 +11 -12 +11 1984 Mondale +1 -12 -9 -13 +3 1984 Reagan +29 +24 +26 -5 +2 1988 Dukakis +7 +7 -8 0 -15 1988 Bush -5 +14 +13 +19 -1 1992 Clinton -11 +8 -1 +19 -9 1992 Bush -13 -15 -13 -2 +2 1996 Clinton +9 +13 +9 +4 -4 1996 Dole -17 -5 -13 +12 -8 2000 Gore +1 +18 +8 +17 -10 2000 Bush +18 +18 +13 0 -5 2004 Kerry +3 -9 -6 -12 +3 2004 Bush -4 +8 +6 +12 -2 2008 Obama +7 +7 +16 0 +9 2008 McCain +1 +9 -3 +8 -12 2012 Obama -3 +4 0 +7 -4 2012 Romney -5 -7 +1 -2 +8 Are Trump and Clinton’s favorability ratings now set? Net favorability numbers are from before and after both conventions each election cycle. Source: CBS NEWS

Jimmy Carter was very unpopular in 1980 before the conventions, but saw his net favorability rise 24 percentage points by the time they were over, basically matching Ronald Reagan’s rating (Reagan’s fell during the convention period). Reagan, of course, went onto win in the fall easily. We probably should have viewed Carter’s improvement skeptically given his job approval ratings still weren’t very good throughout the period.

John McCain suffered a similar fate in 2008. After a strong convention, and the initially successful rollout of his running mate, Sarah Palin, McCain got an 8 percentage point bump; then-Sen.Barack Obama’s ratings, meanwhile, held steady. By November, though, Obama’s popularity had grown, and McCain’s fell back below pre-convention levels. McCain’s favorability may have been undercut by his clumsy response to the worsening U.S. economy, or maybe McCain’s popularity was just artificially inflated and would have reversed to its pre-convention level no matter what.

Of course, sometimes a candidate uses the convention to introduce himself to the country in earnest, and their newfound popularity sticks. George H.W. Bush’s net favorability jumped 19 percentage points by the end of convention season in 1988, and he went on to defeat Michael Dukakis, who had led in the polls before the conventions. The same thing happened to Bush’s son, George W. Bush, in 2004. The younger Bush had been a slight underdog in his race against John Kerry before the conventions, but used the Republican convention stage in New York to take control of the race for the rest of the campaign.

Is Trump the next Bush or the next Carter? The historical record suggests one fate isn’t much more likely than the other. Of the 18 candidates studied here, the pre-convention favorability polls were closer to the final mark in eight cases, and the post-convention polls were closer in seven. (Three times the pre- and post-convention polls matched.)

Favorability ratings can change a good deal from the convention period to Election Day. That is likely to be even more true in 2016; the parties held their conventions much earlier than usual this year, in July rather than late August or early September — so there’s more campaign left to go than is typical post-conventions.

NET FAVORABILITY ABSOLUTE DIFFERENCE FROM FINAL POLL YEAR CANDIDATE BEFORE CONV. AFTER CONV. FINAL BEFORE CONV. AFTER CONV. 1980 Carter -25 -1 -8 17 7 1980 Reagan +12 +0 +11 1 11 1984 Mondale +1 -12 -9 10 3 1984 Reagan +29 +24 +26 3 2 1988 Dukakis +7 +7 -8 15 15 1988 Bush -5 +14 +13 18 1 1992 Clinton -11 +8 -1 10 9 1992 Bush -13 -15 -13 0 2 1996 Clinton +9 +13 +9 0 4 1996 Dole -17 -5 -13 4 8 2000 Gore +1 +18 +8 7 10 2000 Bush +18 +18 +13 5 5 2004 Kerry +3 -9 -6 9 3 2004 Bush -4 +8 +6 10 2 2008 Obama +7 +7 +16 9 9 2008 McCain +1 +9 -3 4 12 2012 Obama -3 +4 +0 3 4 2012 Romney -5 -7 +1 6 8 Average 7.3 6.4 Are pre- or post-convention favorabilities more predictive? Net favorability numbers are from before and after both conventions each election cycle. Source: CBS NEWS

For now, the wisest thing to do is probably to average the pre-convention and post-convention net favorability data. When compared to the campaign’s final net favorability poll, the average absolute error by combining the pre and post-convention polls is 1.9 percentage points lower than the post-convention polls and 2.8 percentage points lower than the pre-convention polls. If that rule works this time, it’s probably not good news for Trump, who was well below Clinton’s net favorability before the conventions and has only roughly equaled hers now.