Assignment  15
Experimental Design

Adapted from the FIRST II Project (www.first2.org)
Images from http://info.hartwick.edu/biology/def_frogs

Goals:

  1. To further develop expertise in experimental design and interpretation of experimental results.
  2. To draw valid conclusions from experimental data and consider their implications for answering broader and future scientific questions.

Objectives: Students will be able to

  • explain an experimental design in terms of its purpose, dependent and independent variables, and controlled variables
  • derive hypotheses from stated research
  • graph experimental data
  • interprete data about the effects of abiotic and biotic variables on frog development
  • state valid scientific conclusions based on experimental evidence
  • make predictions based on experimental data

 

At this point in your science education, you may or may not have had much experience with scientific methodologies.  The purpose of these activities are to give you experience with experimental design, which is a key component of scientific inquiry.  You already started designing experiments through your organismal and ecological research projects.  You will further develop your understanding and skills in this activity so that you are prepared to design more extensive experiments in subsequent activities.  Rather than conducting your own research, you will begin by studying investigations of an unusual phenomenon concerning amphibian deformities.

 


PART A.  What's going on with frogs?

Since 1980, over 40% the world's estimated 6,260 amphibian species are threatened or extinct in almost every part of the world, even in protected wildlife reserves and parks.  In some locales, frog deformities (such as extra legs and missing legs), which usually lead to mortality, are occurring in unusually high numbers.  Imagine that you found the frogs pictured above in a pond on your property.  On further investigation, you discovered that over 30% of frogs in that pond have similar deformities.  How will you determine what caused the deformities?
 
Think INDIVIDUALLY for a minute.

Now brainstorm with your group.

EXPLORE:  Solve the deformed frog problem.
This is a group problem, but each individual is to record everything in his/her notebook. 

Select a recorder for your group.  The recorder is to turn in the group responses to questions marked with an asterisk (*).  Be sure to include the group name, and names of the group members.

In your group, discuss and list possible causes of the frog deformities.  Consider both biotic and abiotic variables (5 min)

Scientists working on the question of frog deformities have proposed several possible causes, four of which are
  • increased UV radiation
  • chemical pollution
  • trematode parasites
  • chytrid fungus

    • Each group has data related to each predicted cause of frog deformities.  These data are based on compilation of results from several experiments, and represent actual trends in current research findings.
    trematode life cycle
    Data sheets:

    Each individual in your group studies one of the four data sheets.   
    1. From the description of the experiment on the data sheet which you have, draw a picture of the experimental design.
    2. Now write a possible hypothesis the scientist was testing with the experiment.

    Groups:  Review each possible hypothesis and design with each other (5 min).
    *Recorder: Record the four hypotheses.

    Each individual in the group (who is now an expert on one experiment) does the following:
    1. Graph the data in a format that will help you analyze it and present it to your group.
    2. Interpret the data.  Write you interpretation.
    3. Answer:  Do these data support the hypothesis which you wrote for the cause of frog deformities in the pond?
    Groups:  What conclusion do you make about the cause of frog deformities in the original pond based on the results from all three experiments?  Explain and support the conclusion.

    *Recorder:  Write your group's responses (5 min).

    EXPLAIN:  Does the conclusion from the research explain the increased frequency of frog deformities in your pond?

    Think about this and then discuss your ideas with your group members.  For example, below are questions to consider if you studied the the effects of trematodes. Pose similar questions for the other data sets to help you determine any effects of each variable studied.
    1. Is there any evidence that trematodes are increasing in frequency?
    2. Is there any evidence that trematodes have developed a mutation that enables them to infect more frogs?
    3. Do any data from the experiments show that frogs are becoming more susceptible to parasitic infections?
    4. How could global change influence the susceptibility of frogs to parasitic or fungal infections?
    *Recorder:  Include key points of the discussion in your group report (5 min).

    FURTHER EXPERIMENTATION:  Synergistic effects of two variables.

     Hypothesis:  The effects of atrazine and trematodes together on the proportion of frogs that exhibit deformities are greater than the effects of either atrazine or trematodes alone.

    Experimental Design:
    • frogs reared in ponds with atrazine or without
    • cages with trematodes or without
    • 3 ponds per treatment; 4 replicate cages per pond
    • 10 tadpoles raised to frog stage per cage
    • data are averages of the 4 cages in a pond

    Results:

    Treatment % of frogs with deformities
    No Atrazine/No Trematodes 0
    No Atrazine/No Trematodes 0
    No Atrazine/No Trematodes 0
    No Atrazine/ Trematodes 5
    No Atrazine/ Trematodes 5
    No Atrazine/ Trematodes 10
    Atrazine/No Trematodes 0
    Atrazine/No Trematodes 0
    Atrazine/No Trematodes 0
    Atrazine/ Trematodes 22
    Atrazine/ Trematodes 30
    Atrazine/ Trematodes 2

    Data estimated from Figure 1 in: Kiesecker JM. 2002. Synergism between trematode infection and pesticide exposure: A link to amphibian limb deformities in nature? Proc. Nat. Acad. Sci. 99(15): 9900-9904.

    Discuss the results in your group.

    *Recorder:  In one sentence, interpret the results in the context of the stated hypothesis.  Use the header:
           Results of experiment with two variables

    FOR FURTHER THOUGHT: What are the implications of these results on human health? What are your predictions?

    global disease

    PART B.  The Frog Pond Tutorial

    Next you will further investigate frog deformities using a simple simulation to conduct experiments.      Simulations do not replace real experiments.  Rather, simulations are a valuable educational tool because they allow you to conduct multiple experiments in a much shorter period of time than if you were working in the field or laboratory.  In the Frog Pond simulation, you will investigate whether genetic or environmental factors cause variation among individual frogs.  Your Professor will provide you with instructions for using the Frog Pond Tutorial, which is located in your course manual.

    PART C.  Understanding Experimental Design

    Learning to conduct scientific experiments that are rigorous in their design and provide reliable evidence that supports or fails to support a hypothesis(es) takes practice. You probably already have some experience with experimental design from high school. As you are already aware, designing experiments is a critical component of this course. Here you will review the basics and expand your expertise in experimental design. You will use a simulated disease system and personalized feedback to overcome any confusions you still have about experimental design. You will reinforce the skills you are developing in experimental design and summarizing and interpreting experimental results. The main concepts that you will study here are systematic variation, control treatments, replication, and scope of inference. Follow the instructions provided by your professor and complete the Understanding Experimental Design module.