The week's biggest political news. Nope, not conventions.

Friday's unemployment report will be crucial for both presidential candidates. If the rate of unemployment drops, President Obama’s claim that we’re on the right track gains credibility. But if these numbers are moving in the wrong direction, Romney’s claim that  the nation needs a new start may gain traction.

The biggest political news this week won’t be the Democratic convention. It will be Friday’s unemployment report.

If the trend is good — if the rate of unemployment drops and the number of payroll jobs is as good if not better than it was in July — President Obama’s claim we’re on the right track gains crucial credibility. But if these numbers are moving in the wrong direction, Romney’s claim the nation needs a new start may appear more credible. 

I don’t recall a time when these jobs numbers, compiled monthly by the Bureau of Labor Statistics (a highly professional group whose findings are completely insulated from politics), were as politically significant as they’ll be this Friday, and the first Fridays in October and November. 

Yet these numbers are really crude approximations. They’re adjusted for seasonal variations — based on historical data that may have less significance today, when the economy is still struggling to emerge from the worst downturn since the Great Depression. The numbers are also subject to corrections and revisions later, as more data come in. 

But perhaps the biggest flaw — and irony — is that when and if jobs really do start to return, many of the people who had been too discouraged to look for work start looking again. And when more people are looking, the rate of unemployment rises — because that rate is based on the percent of Americans actively looking for work. Those who have stopped looking aren’t counted.