A president’s overall approval rating is a very important number -- perhaps the most important single number in politics -- as this week’s fracas over whether President Obama’s numbers are up, down or holding steady suggests.

But looking only at the approval side of the equation can be deceptive. In particular, overlooking the big differences in “no opinion” answers can mislead poll watchers.

To aid the ongoing poll parsing, I’ve included two charts below. The first is an interactive that allows you to see how six high-quality national polls released this week stack up when it comes to approval, disapproval and don’t know responses on Obama’s overall job performance.

Each of these polls includes cell-phones in their samples. Both March (this week) numbers and those from a month ago (or so) are included, broken out by group.

The second chart shows the percentage point change within each group, across surveys. Poll results are commonly mis-interpreted by focusing on changes “on the margin” as opposed to examining the type of changes displayed here.

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