Do you ever check on a cake while it's baking? Taste the sauce while it's simmering? Weigh yourself while you are trying to drop 10 pounds? Of course you do. We know what the desired outcome is, yet we generally check on progress along the way to see if things are on track or if adjustments need to be made.

This reasoning surely applies to student progress, where ongoing assessments give teachers important information about student strengths and weaknesses. But not according to United Teachers Los Angeles, which has called for a teacher boycott of district-required "periodic assessments," calling them costly and counterproductive.

The union's objections are based on the notion that these assessments are just more teaching to the test, and that getting rid of them would save millions that could be spent elsewhere. To be sure, many districts squander instructional time on dubious test prep practices like bubbling strategies. But any teacher who thinks formative assessments are a waste clearly fails to understand how student data can be used to differentiate instruction. If your plan is to keep doing what you are doing regardless of student performance, then assessment is pointless.

"The pig does not get fatter when you weigh it 10 times a day," says union president A.J. Duffy. Indeed it doesn't, if you keep feeding it the same diet.