Lesson 4

Which Picks Up More?

Est. Class Sessions: 4–5

Developing the Lesson

Part 2. Collecting and Recording the Area Data

Discuss Finding the Median Area. Discuss with the class the need to measure more than one spot for each brand of paper towel. The data table on The Better “Picker Upper” Lab pages in the Student Activity Book provides room to record the results of three trials and their median value. For a discussion of finding the median, see Lesson 3 Boo the Blob.

Ask:

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  • Why is it a good idea to find the area of three spots on each brand of paper towel, instead of just one? (We might make mistakes. We might not hold a fixed variable constant, and if we found the area of just one spot, it could be inaccurate. See the Content Note about multiple trials.)
  • Why find a median area for the three spots? (A median will give us an average, or typical, area of a spot made on that brand of paper towel.)

The groups are now ready to collect data about each paper towel brand. Distribute the lab materials you organized for each group of 4 students: an eyedropper, one sheet of each brand of paper towel, scissors, small container of water, and books or geoboards for drying the paper towels. Students will collect and record data on the data table provided in Question 3 of the Collect section of The Better “Picker Upper” Lab pages in the Student Activity Book.

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Multiple Trials. If you ask third-graders why it is a good idea to find the area of three spots on each type of paper towel, they often answer, “We might make a mistake.” They are correct. In this experiment, there are several ways that errors can be introduced. First, as introduced in Lesson 2 Measuring Area, counting square centimeters does not allow us to measure the area of curved shapes (like the spots) exactly. Even if students were able to trace, cut, and count the square centimeters with maximum precision, they would still only be able to measure the area to the nearest square centimeter or half of a square centimeter. Second, it is not always possible to keep all the fixed variables perfectly constant. For instance, students may not be able to keep the drops the same size or to place each drop exactly where they placed the previous one. Scientists use data from multiple trials to average out the inevitable errors in an experiment and, at the same time, to check for large errors in measurement or in controlling fixed variables. If the error is too large, scientists do additional trials.

Check Student Data. While students are gathering and recording data, check the results of each group. Although we expect a certain amount of error, the measured area of the three spots for each type of paper towel should be relatively close. In Daniel's data table shown in Figure 4, the results of the three trials for both Brand 1 and Brand 2 are as close to one another as we can expect.

However, the data for Brand 3 looks suspect: the area for the first trial is almost double the area found in the third trial. These differences provide an opportunity to discuss the possibility of making mistakes—perhaps Daniel did not trace or cut out one of the spots as carefully. This is also a good time to point out problems that can occur if the fixed variables are not actually held constant—perhaps the first spot contained too many drops. If possible, this group should perform a fourth trial and use this data to judge which trial contains the most error and replace it with the new trial.

Student Guide:
Student Activity Book: 160
Answers
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SAB_Mini
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Master:
Daniel's data table
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