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13 November 2014

Why I make students graph data as they collect it

When I run a laboratory exercise, students are required to "graph as they go" -- that is, data are not written in a table for processing later, but are plotted directly and immediately on a graph.  The inevitable question, from students and fellow teachers, is why?  I mean, physics data don't go stale.  The graph is gonna look the same if it's plotted tomorrow.  What is the advantage to insisting on a live graph during the laboratory exercise? 

The most important advantage has to do with how students understand experiments. A data table just looks like a bunch of random numbers, both to students and to experienced physicists.  It's when the data is put on a graph that patterns can be seen and understood.  By graphing as they go, students develop for themselves an instinct about how much data is "enough," whether the full parameter space is covered, what further data is useful, etc.  

Science teachers are always talking about avoiding a cookbook mentality in the laboratory, in which students mindlessly follow directions trying desperately to get the "right" answer.  Well, here's one way to get students to connect intimately with their data -- as they see the graph develop, they think about and process how the data connects with the physical experiment.  They wonder whether the graph will end up straight or curved, they construct hypotheses in their heads which are borne out or not by the graph.  

The practical advantage of "graph as you go" is that students don't write down a bunch of numbers and assume they're done.  I get pushback if students have sat at their desks to construct a graph, then are told "ooh, let's get some further data in this region of the graph."  Aww, man, I thought we were finished.  I even put the track away.  Do we really have to get everything out again and do more?  Can we just do ONE more point, or do we have to do a lot?  Grrrr...

If all data is going on the graph right away, I can walk around the room and suggest right away how their data collection process is going.  Everyone expects and welcomes my input as part and parcel of the lab course.  Lab becomes about producing beautiful graphs, not about getting done and away from the annoying physics teacher.


  1. Great post! I love how you've really thought about how this can affect the students' learning in lab. It reminds me of a conversation I had with a student who had collected tons of data. His first stab at a formal presentation was to list the data. I told him that no one can make sense of a list of data. You have to plot it, or it won't make sense. I like this suggestion (plot right away) even more. I think the same is true for automated data collection. In Vernier it naturally does this. But in some incarnations of using Arduinos, the data comes as a list that needs to be copied and pasted elsewhere for plotting. I've noticed that using the latter isn't always as good as the former.

    My question: If a students starts to see a trend that matches their intuition, will they stop if the data starts going away from the trend and say "I'm done"?

  2. Thanks, Andy... early on, students will tend to stop work and argue with their partner about data that doesn't match their intuition. I have to come around and crack the (figurative) whip to get them going again. As the year progresses, they understand more and more how experimental data behaves. They'll change from "I'll bet this is a line" to "well, if this is a line, data in this range will fill in right here; but if this is a curve, the data in this range will fill in over here instead." That's good physics.

  3. Greg, How do you have your students decide on a scale if they are graphing their data as they collect it?

  4. Alden, I allow them to measure a point or two for the purpose of determining scale. Or, they can estimate what they think the scale should be. A lot of good discovery comes from marking a horizontal axis, say, from 0-5 seconds, then finding as they graph that the ball never takes more than 0.80 s to roll down. Regraphing by hand means so much more than simply zooming on an ipad...