Evaluate Results
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Identify Variables
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Develop a Hypothesis
Draw Conclusions
Part 1 of 6
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Introduction
Many factors play a role in good experimental design and evaluation of results. This tutorial will demonstrate the importance of proper set-up, quantitative measurement, and avoiding bias in drawing conclusions about a hypothesis. If this is your first time through the tutorial, following the steps in order by clicking NEXT on the bottom right will provide the most logical delivery of material.Instructions Select a button from the left menu to skip ahead if you are returning to this tutorial or wish to review certain steps.If you encounter any unfamiliar terms or are uncertain about the scientific process, refer to your lesson and textbook resources for more information.
The following is a hypothetical scenario, serving as the backdrop for this tutorial on experimental design in the scientific process. You are sitting on a bench at a local city park, and notice a mother and child on a nearby bench. The child is crying and has a slightly bleeding scratch on his forearm. You observe the mother open a small bottle of hydrogen peroxide and pour the solution over the scratch. The liquid over the wound forms tiny, white bubbles that look like foam. This captures your interest, and you decide to investigate the situation.
Experimental Design
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You begin by asking a good question: What is the substance forming gas bubbles on the surface of a wound after hydrogen peroxide is applied?
To find the answer, you collect more information. Here is a summary of the information you found:An enzyme called catalase is produced by living cells. Catalase speeds up the breakdown of hydrogen peroxide into oxygen gas and water by the following reaction: 2H2 O2 --> 2 H2O + O2 A cut or scrape is formed by damaging surface blood vessels and skin cells so that the molecules inside, including enzymes such as catalase, are released. All enzymes have optimal conditions that function, including temperature and pH.
Based on the information collected, you decide to scientifically investigate the formation of oxygen gas. You perform some additional background research and discover that baker’s yeast (Saccharomyces cerevisiae), produce the enzyme catalase. Yeast are single-celled fungi. Baker’s yeast are used to help dough rise during bread-making and in fermenting alcohols such as beer and wine. You read about others who have performed experiments to demonstrate formation of oxygen gas by hydrogen peroxide in the presence of yeast by collecting the forming gas into a balloon. This sounds like a fun experiment, so you decide to test it for yourself!
Wanting to try something new, you decide to write a hypothesis about something you read in the background research stage. All enzymes have optimal conditions that affect function, including pH. Based on this information, you choose the following question: What are the optimal pH conditions for catalase enzyme?
Part 2 of 6
What are the optimal pH conditions for the enzyme catalase? Before identifying variables and writing a hypothesis, you collect the following additional information:Most enzymes have evolved to function optimally in environments similar to their natural environment. Some yeast species are a common inhabitant of women’s reproductive tracts. The female reproductive tract is usually pH ~4 (somewhat acidic). Baker’s yeast display optimum growth at pH values between 4 and 5.
Your question is: What are the optimal pH conditions for the enzyme catalase? Based on the information collected, write your hypothesis on a piece of paper for reference later. Is your hypothesis testable and falsifiable? Does the hypothesis propose a general, rather than specific, explanation? Does your hypothesis allow you to make multiple predictions for testing? Let’s proceed with the following hypothesis, which may be similar to yours: Catalase functions optimally at pH values close to 4.
The hypothesis you will be testing is: Catalase functions optimally at pH values close to 4. Let’s make some predictions using the hypothesis. Although many possibilities exist, leading to many different experiments, the following are one set of possible predictions:Catalase activity will be higher at pH 4 than at pH 7 (neutral).Catalase activity will be lower at pH 2 than at pH 4.In the presence of catalase, production of oxygen gas from hydrogen peroxide will be higher at pH 4 than pH 7.In the presence of catalase, production of oxygen gas from hydrogen peroxide will be lower at pH 2 than pH 4.
Now that we have identified a set of related predictions, let’s design an experiment. Based on your background reading, you plan to use the following procedure for measuring catalase enzyme activity:Place 10 mL of hydrogen peroxide into a test tube.Add 5 mL of buffered solution to adjust pH to desired level.Add ¼ teaspoon of yeast to the test tube and immediately attach a balloon to the top of the tube to capture all gas produced.Mix thoroughly while observing for 20 seconds.Using a string or other flexible tool, measure thecircumference of the balloon 30 seconds after addition of yeast.Record circumference of balloon to determine relative oxygen gas production.
Correct. pH is the variable that will be altered in the experiment. Remember that an experiment should have only one independent variable.
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rate of oxygen production
Before we choose conditions, let’s identify the independent, dependent and controlled variables.
test tube size
person conducting the experiments
balloon choice
time before measuring
1. Select the independent variable(s) for this experiment: Choose one or more options. Select the boxes to check or uncheck your answer(s). Select NEXT when you have identified the correct answer(s).
Instructions
catalase enzyme concentration
pH
Part 3 of 6
temperature
amount of yeast cells used
Incorrect. The independent variable is the one we change to determine the effect on the dependent variable. Try Again.
rate of hydrogen peroxide depletion
2. Select the dependent variable(s) for this experiment: Choose one or more options. Select the boxes to check or uncheck your answer(s). Select NEXT when you have identified the correct answer(s).
Correct. The rate of oxygen production is the variable being measured when the pH is changed. The experiment is testing if the rate of oxygen production “depends” on the pH value. Remember that an experiment should have only one dependent variable.
Before we choose conditions, let’s identify the independent, dependent and controlled variables.
3. Select the controlled variable(s) for this experiment: Choose one or more options. Select the boxes to check or uncheck your answer(s). Select NEXT when you have identified the correct answer(s).
Incorrect. One or more of your answers is a factor that will change during the experiment. Try Again.
Before we choose conditions, let’s identify the independent, dependent and controlled variables.
Correct. Any factors that may influence the dependent variable must be kept constant for each experiment. If these factors vary, any change in the dependent variable may be due to these factors instead of the independent variable. Are there any more?
You have the time and materials to test six samples the first time you perform the experiment. You'll test the conditions in the table below. When you are ready to begin, select the Test Samples button.
Part 4 of 6
Test Samples
Choose Conditions to Conduct an Experiment Now that we have identified the variables and predictions for the experiment, we are ready to begin.
Yes
Draw conclusions.
No
Are you sure you have enough data to draw a conclusion about your hypothesis?
Here are the test results for the six conditions. Are you ready to draw conclusions? Based on your results, what do you want to do next?
Part 5 of 6
Repeat the experiment to collect more data.
Here is a summary of testing each condition six times. While conducting the experiment you might have chosen to stop testing certain conditions by drawing conclusions about those conditions, but for this activity we will provide results for six tests on each of the six conditions.
With Yeast, pH= 7
Incorrect.
No Yeast, pH= 4
Here is a summary of testing each condition six times. While conducting the experiment you might have chosen to stop testing certain conditions by drawing conclusions about those conditions, but for this activity we will provide results for six tests on each of the six conditions.
To look back at the data you collected, select:
Correct.
Draw Conclusions Based on Objective Data After evaluating the results data, what is the optimal condition for oxygen gas production?
View Data
Part 6 of 6
According to the data, the rate of oxygen production as measured by the increase in balloon size was greatest at pH 7. Because catalase is the enzyme that catalyzes oxygen production from hydrogen peroxide, this experiment supports the conclusion that catalase activity is optimal at pH 7. Additional experimentation should be done to confirm this result by testing catalase activity in other pH conditions.
No Yeast, pH= 7
No Yeast, pH= 2
With Yeast, pH= 2
With Yeast, pH= 4
Your response indicates that you may not have collected enough data to determine the best conclusion. When conclusions are drawn too early, selection bias can sometimes lead to the wrong answer. If only one or a few conditions are tested, the experimenter may miss key data that could affect analysis of the results. Alternatively, conclusions drawn too early may suffer from sampling error. When sample size is small, random variation in procedure or other conditions causes variations in the data that can affect conclusions. Testing each condition several times can reduce sampling error caused by these uncontrolled variables such as not getting the balloon onto the test tube quickly enough in one sample compared to others.
Here is a summary of testing each condition six times. While conducting the experiment you might have chosen to stop testing certain conditions by drawing conclusions about those conditions, but for this activity we will provide results for six tests on each of the six conditions.
You have arrived at the most appropriate conclusion for the experimental data you collected. A good scientist is willing to discard the hypothesis in the face of data that did not support it. The next step might be to propose a new hypothesis and design more experiments!
Partially
The experiments were inconclusive.
No
If sufficient data was collected during repeated experiment attempts, you should have found that the results do not support the hypothesis that catalase functions optimally at pH values close to 4. If your data indicated that pH 4 is optimal, or that multiple conditions provided the same level of enzyme activity, consider whether you encountered selection bias or sampling error in your experimental design and data collection. To avoid selection bias, always include a control group for each condition tested and test as many different conditions as possible. To avoid sampling error, test each condition as many times as possible and average the results to determine the difference in the dependent variable due solely to the independent variable and not to random variation.
Was your hypothesis supported by the experimental data?
Was a control group included in the experimental design?To avoid selection bias, did the experimental design include:a. An unbiased way to collect data such as a quantitative measurement or evaluation rubric (list of characteristics/ events compared)?b. Many different conditions to avoid skewing data towards the expected result?To avoid sampling error, did the experimental design include repeated testing of each condition to ensure a large sample size?
You have successfully worked through the scientific process to perform and analyze experimental data. When performing experiments or reading news reports about experiments, consider the following as you evaluate the results and draw conclusions.
Congratulations! You have successfully designed and executed an experiment using the scientific process! The original hypothesis was proven false, but you may have formulated an alternative hypothesis in response to the new experimental data. Below are some suggestions for follow up experiments to test a new hypothesis. Can you think of others? Repeat experiments with additional pH values to determine a more precise optimal pH or pH range.Repeat experiments with catalase from an alternate living organism to determine if optimal conditions vary between organisms.Repeat experiments to increase sample size and reduce the error caused by sample or experimental procedure variations. •