Estimating Turtle Size and Age

Grade Levels: 5 - 12

This activity is very similar to the Long-Lived Turtles lesson except that it adds the concepts of sampling error and variability in scientific data. It is appropriate for more advanced students who are learning about research and ways to analyze data.


Students will learn how to estimate age and size of turtles, and understand variability in scientific data.


  • Live turtle or turtle shell
  • Flexible transparent ruler with millimeter scale
  • Copies of two diagrams: Measuring Turtle Length and Estimating Turtle Age from Scutes
  • Small sheets of paper on which to record measurements
  • Calculator
  • Blackboard with chalk
  • Hygienic wipes or antibacterial soap


  1. View background information on Reptile and Amphibian Life Span and Life History.

  2. Select one turtle or turtle shell.

  3. Identify the parts of the shell including carapace (back), plastron (underside), and scutes (scales).

  4. Refer to the diagram to show students how to measure the length of the turtle's plastron along the midline from one end to the other. Then demonstrate how to measure the carapace length using first the straight-line method and then the curved-line method.

  5. Have students measure the length of each turtle shell and record their measurements.

  6. Point out the lines on the scutes of either the carapace or plastron. Use the diagram to demonstrate how to count the lines on one scute. The number of lines on each scute represents an estimate of the age of the turtle.

  7. Have each student independently measure the turtle's carapace length and record the measurement on a piece of paper. Be sure that everyone uses the same method, either straight-line or curved. Also have each student count the number of lines on a scute and record the estimate of the turtle's age.

  8. Assign two people the job of reading the measurements off the slips of paper and recording them in tables on the board. Did everyone use the same units of measure? If not, convert the values to the same units. Calculate the average of the measurements for the length and age of the turtle.

  9. Identify the maximum and minimum values reported for length and age of the turtle. These values will give the students an idea of the variability involved in measuring. Have the students talk about exactly how each person measured the shell and compare techniques. The variability in how they measured the shell is a type of sampling error. It always occurs in sampling and can be a problem for scientists trying to gather exact information. Some of the sampling error is due to mistakes, whereas some is due to differences in researcher technique. Discuss with the students how they might reduce this sort of variability. For example, teachers might provide clearer directions for taking measurements, or researchers might practice on a model or separate animal first.

  10. Ask the students to identify other kinds of variability that might affect the accuracy of their results. For example, turtles may not add exactly one growth line per year, so counting the lines may not give an accurate measure of age. Also, as turtles get older, their shells often get so worn that some or all of the growth lines may be obliterated.

  11. Have the students take the entire range of sizes recorded and divide it into five equal intervals. Draw a graph with the size intervals on the x-axis and numbers on the y-axis ranging from zero to the total number of students. Have the students plot the total number of measurements that were recorded by them within each of the size intervals. This is a graph of the distribution of the variability of the measurements. If all the measurements were the same, there would be only one point on the graph. Usually, however, you will see a bell-shaped curve, with the most measurements occurring at the average size and a few measurements near the minimum and maximum sizes. This is the famous bell curve of a normal distribution. In general, the more measurements you have, the smoother the curve.

  12. Finish by talking about why there might be more measurements near the average size. Much of the statistical analysis that researchers perform begins by examining the information in the same way that the students have just done.
Excerpted from Hands-On Herpetology.

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