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Observational and interpretive skills  

(Competency E2.1 from the SFFP Report)

Sussing out errors in data

Every laboratory makes occasional errors; and in communicating data from one group to another there is always the possibility of errors in transmission. One of the most important skills a scientist can develop is an understanding of how to look at data for consistency and accuracy.  What are the clues that tell you that there might be something wrong? (Metacognition!)

Here are some example problems which deal with errors and apparent errors in data.

1. Grand Jete -- If one picks a fixed point on the dancer, follows it through the leap, and fits it with a parabola, one finds that the acceleration of gravity, g, is about 20% less than it should be.  What's wrong? In this case there is no error in the data. The error is in the assumption that each point on an object that is changing its shape accelerates with g.  Only the center of mass does that.
2. Listeria --  In this problem we follow the motion of a single bacterium in the video as it moves in a cell. We find that it stops and starts in a jerky motion. Since we have a theoretical picture of how it is moving -- by actin polymerization -- we expect a "ratchet-like" jerky motion. But is that what's happening? The video comes off the internet and we don't have details. Sometimes, videographers put in duplicate frames to slow things down and smooth them out. Is that what's happened here or are they really ratcheting?
3. TA on a skateboard -- Another video problem like Grand Jete.  A TA sliding at a constant velocity throws a tennis ball straight up and catches it.  Following the ball and fitting the y-t motion with a parabola gives a result that is about 20% off.  Since the ball doesn't change its shape, the answer to problem 1 can't be the reason. Perhaps the frame rate that was quoted was wrong.  What would the frame rate have to be to make g correct? (Also involves scaling since [a] = L/T²)
4. Motion detector -- A problem analyzing errors in the motion detector due to pileup and changes in the speed of sound.

Joe Redish 9/6/11