Evaluating Existing Experimental Designs

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Students will be able to identify variables in existing experiments and exercise the conceptual skills they will use to design their own experiments in the next lesson.

Big Idea

Proper application of the scientific method requires careful experimental design and control of variables.


This lesson builds upon the previous lesson about the scientific method and allows students to put the vocabulary and concepts they learned into practice.

Since experiments can be a time consuming process in a science course, I find it worth the time early in the year to have students evaluate flawed experiments to understand the importance of careful experimental design.

Regarding the targeted standard of the lesson, this lesson is focused on analyzing informational narratives which present a body of evidence that students will use to support conclusions about the efficacy of a given experiment.  By close reading and analysis of scenarios describing existing experiments, students should be able to reflect on the necessary measures they'll need to take to ensure that their own experiments they will conduct later in the course are opportunities to partake in the process of science rather than just learn about the body of knowledge that results from that process.   

Warm Up

15 minutes

I begin the warm up by distributing the “scientific method worksheet”*

This worksheet gives students the opportunity to evaluate experiments and practice identifying hypotheses, independent and dependent variables, control and experimental groups, and drawing conclusions based on the evidence supplied in the scenarios.

It's important that all students are on the same page with this activity from the beginning, so this warm up is a good opportunity to make sure students are familiar with the terms and concepts introduced in the last lesson before they begin working on the activity in earnest.


After distributing the worksheet, I ask students to carefully read and answer the first scenario (regarding workers at an automobile factory).  I let them know that for most of the class, they will be working with their partner, but for the warm up I ask them to read and work by themselves for about 5 minutes.   Then I ask them to spend 3-5 minutes comparing and discussing their answers with their partner.

Following their own work on the problem, I have the bullet points corresponding to the required information for the scenario on the board.  I ask for volunteers to help fill in the information and I write it down as they offer the info.  


There is a teacher's key resource for all the scenarios in the review section of this lesson with suggested answers, but I include the first scenario here to guide the warm up: 

1.  The manager of an automobile factory feels that his workers aren’t working as well as they could and thinks that he can improve productivity by having workers drink Awesome Drink Energy Juice.  He splits his workers into two groups, group A and group B:  Group A receives the Awesome Drink Energy Juice and Group B does not.  He asks them to assemble as many cars as possible in one hour.  At the end of one hour, Group A assembled 27 cars and Group B assembled 41 cars.              

  • What was the factory manager’s hypothesis? That the Awesome Drink Energy Juice would increase productivity
  • What was the independent variable? The Awesome Drink Energy Juice
  • What was the dependent variable? The number of cars assembled by each group
  • Which group was the control group and which group was the experimental group? Group B did not receive the juice, so they were the control.  Group A received the juice so they were the experimental group.
  • What conclusion should the factory manager make from his experiment?  That the juice did not improve productivity, in fact, it seemed to reduce productivity. 
  • How could his experiment be improved? Answers will vary, for example: Repeat the experiment with the same groups of workers, but switch which group receives the juice… perhaps Group B is made up of naturally faster workers.  Or, have Group B drink water during the experiment, perhaps the act of drinking somehow slowed down Group A.


Hopefully as students answer the required information and provide different ways the experiment could be improved, students will be well prepared to think creatively about the scenarios in the rest of the worksheet.   

Guided Practice

30 minutes

After the warm up, students begin working with their partners on the worksheet.*

As they work, I walk around the class offering help where it is needed.  One of the first things to do is refer back to the note sheet that was distributed with the powerpoint from the previous lesson on the scientific method so they can use this as a resource for the vocabulary.  

Occasionally, groups may get stuck (or complacent) and it may take more of an involved discussion for a group to approach a question more substantively and creatively.  A strategy for this situation would be to use some guiding questions (e.g., “what does the factory manager think the drink will do to the workers?”, “Was that the only variable?”, “Was he looking for quantitative data or qualitative data?”, etc.).  For many students at the high school level, it may just be necessary to quiz them with questions such as, “what is the independent variable?”, “can you explain how that result disproves her hypothesis?”, etc.  When in doubt, press them to add more detail to their explanations.  

The last part of the worksheet allows them to write down the parameters of an experiment of their own design with a clear goal.  This is valuable practice and preparation for the upcoming lesson where they will design and conduct their own experiment.

If your class moves quickly through this worksheet, that's fine.  Move ahead to the review section which offers plenty of opportunities for a more in-depth discussion.  

*this worksheet is based, in part, on "Identify the Controls and Variables", a worksheet on biology corner.com using Simpsons characters.  The original is arguably more fun, so use it if you prefer.  However, the worksheet provided specifically for this lesson contains an environmental science issue that offers a preview of many of the issues and concepts that will be explored in more detail later in the course.


15 minutes

Like I mentioned in the previous section, don't worry if your students finish the worksheet with seemingly too much time to spare, there's plenty of "meat" left in the review and it could really take as long you and your class would like to talk about it. 


The "Teacher's Answer Key: Evaluating experiments worksheet" resource attached to this section can be used as students work on providing the basic information (independent and dependent variables, hypothesis, etc.) for each scenario, but I think it is most useful here as a way to guide the discussion of how the experiments could be improved.  

Again, the main idea of this lesson is for students to understand the considerations built into a well-designed experiment so that they can apply those same kinds of considerations to their own experiments.  


Scenario 2 is fairly straight forward, but I would encourage teachers to make sure students understand that the natural variation of people in separate groups introduces unknown variables.  These variables can be controlled, however, with good experimental design (in the case of this scenario, having each group try the physical challenge under both conditions to account for natural differences in strength that may not be due to the independent variable).  

Scenario 3 deals directly with an environmental science issue and is intended to facilitate a broader discussion of the specific challenges of environmental scientists, mainly that natural populations don’t exist in aquariums being fed fish food.  Populations exist as part of a complex ecosystem with many other components and variables than accounted for in the team’s simple model.  (e.g., perhaps the rising temperature doesn’t directly affect the fish, but may affect their food, or predators, or breeding areas, or some other aspect of their habitat that will ultimately affect their population)

Question 4 is a follow-up to scenario 3 and is intended to generate a discussion about the importance of peer review.  Hopefully students will mention that because there were some obvious flaws in the design of the experiment, it would be irresponsible to publicize the information without first consulting with other scientists.  High school students interested in studying environmental science don’t live in a vacuum.  They are aware that global warming is a highly politicized issue and are likely aware that dissemination of misconceptions and misinformation can hamper meaningful action.  Since this question has an ethical component (what SHOULD they do?), this may be ripe for a short debate or vote, asking students of differing opinions to explain their reasoning.  

Scenario 5 is fairly straight forward in terms of the scientific problem being investigated, but since it really ask students to not only improve an existing experiment but to design their own from scratch, this is a great opportunity for different groups to share their plans.  If you have a document reader, it might be good to have a few groups show the basic procedure they came up with. Otherwise, you could have them fill in the required information on separate parts of the board or just share their ideas verbally with the class.

A superior alternative, if time allows, is to model peer review and have groups of partners exchange their work with another group.  They could then compare how their designs were similar, how they were different, and how they could be improved.  After the "peer review", they could discuss the results of the review with the other group.  Following this, you might ask for a few volunteers to share the highlights of their discussion.  A fun "wrap-up" might then be for the class to vote which group in the class had the design for the fertilizer experiment.