Hillary Clinton has opened up a 7-point lead over Donald Trump in an online poll that seems to reflect a 'bounce' for the former secretary of state after she wrapped up the Democratic nomination last week.

But the weekly tracking poll, from NBC News and the Surveymonkey company, included the opinions of 7 per cent more self-identified Democrats than Republicans – the same margin as the poll's topline result.

That raises questions about the 49-42 result.

BUILT-IN ADVANTAGE: Pollsters with NBC and Surveymonkey included more Democrats than Republicans in their latest offering because federal government data suggest there are more of them in the U.S. – but no one knows who will actually show up and vote in November

ANOTHER DIG: Trump has a love-hate relationship with polls and typically ignores those he doesn't like

Pollsters generally adjust their results to reflect the likely makeup of the electorate, massaging data to match the expected turnout among demographic groups.

In the case of Tuesday's poll results, however, federal government statistics – not voter surveys – were used to arrive at a conclusion that there are far more Democrats than Republicans who will vote in the fall.

'Data for this week has been weighted for age, race, sex, education, region, and voter registration status using the Census Bureau and Bureau of Labor Statistic’s Current Population Survey,' the pollsters say in the fine print, 'to reflect the demographic composition of the United States.'

The most recent Gallup polling on the subject, published in January, found the Democrats had just a 3 per cent advantage nationally. In that same survey, Republican-dominated states outnumbered those populated mostly by Democrats, by a 20-12 margin.

The raw information collected by Surveymonkey each week has generated a distribution of Democrats and Republicans that appears random, and it's unclear how much it was tweaked to fall in line with Census and Labor Department numbers.

The polling data show nearly all the organization's results this year have 'oversampled' Democrats. But one week's results, published in January, included more Republicans.

Adding the sampling disparity to the poll's margins of error, each of the last six week's results can be seen as a dead heat if Democrats and Republicans were to come to the polls in roughly equal numbers this year.

Discerning who will show up and who will stay home is a polling exercise that can give statisticians the night-sweats.

ILLUSION? Hillary has a strong lead when NBC's polling experts announce their conclusions

OVERSAMPLED: Far more Democrats than Republicans were asked for their opinions

The two most recent presidential elections trended heavily in the Democrats' favor as Barack Obama built and then replicated a coalition of liberals and heavily leveraged African-American voters to win.

It's unclear whether or not Hillary Clinton can match Obama's turnout figures among ethnic minorities. Failing to mobilize them in November would mean this year's pool of voters would look dramatically different from the 2012 electorate.

Clinton's 7-point lead grew from 4 just a week earlier, buffeted by her success in finally collecting enough Democratic National Convention delegates to claim her party's presidential nomination.

But when Surveymonkey included the names of a pair of third-party candidates in the mix, Clinton's edge was back to just 4 points.