1. Introduction: the Scientific Method
The scientific method is how scientists formulate and test hypotheses. Typically, this method is taught as a series of intellectual moves. In practice, the way that scientists develop ideas, test them and draw conclusions can vary.
To assist you in the inquiry that you’ll be doing in your AP Biology course, we’ll start by introducing the traditional model of the scientific method. Then we’ll look at how scientists set up controlled experiments in order to test their ideas.
Drag the steps below, listed in alphabetical order, into an order that matches the steps described in the table.
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[h] Scientific Method Matching
[q labels = “top”]
| __________________________________ | This is where it begins: Sensing the world and noticing patterns and relationships |
| __________________________________ | This stage involves making an educated guess that includes a prediction, |
| __________________________________ | This phase involves a structured form of observation that allows you to examine one thing at a time. |
| __________________________________ | This last stage involves answering questions such as 1) Was the hypothesis correct? 2) Are there other lines of evidence that point in the same direction? |
[l]Drawing conclusions
[f*] Correct!
[fx] No. Please try again.
[l]Formulating hypotheses
[f*] Good!
[fx] No. Please try again.
[l]Making observations
[f*] Excellent!
[fx] No, that’s not correct. Please try again.
[l]Performing experiments
[f*] Great!
[fx] No, that’s not correct. Please try again.
[/qwiz]
2. Interactive Reading: A Case Study: The link between cancer and smoking. Initial observations
To learn about the scientific method and experimentation, we’ll look at a simplified history of the discovery of the link between smoking tobacco and cancer. For a detailed view of this story, follow the links to tobaccocontrol.bmj.com at the end of this tutorial. Much of the information below comes from that site.
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[h]Interactive Reading: The Link between Cancer and Smoking
[i]Carefully read what follows, dragging in the words on top to the right place.
[q labels = “top”]
It hasn’t always been known that smoking tobacco caused lung cancer. In the 1500s, tobacco was praised for its supposed health benefits. Lung cancer itself was once extremely _______. But mechanical production of cigarettes, free distribution of cigarettes to soldiers during World War 1, and mass marketing caused a global lung cancer ____________ that began in the 1900s and continues today.
The first observations of the connection started around 1900. The key observation was the rise in _______ cancer rates. Among the first to notice this connection was a German medical student, Hermann Rottman, who noticed higher rates of lung cancer among German __________ workers. Rottman suspected that exposure to tobacco dust was causing cancer.
[l]epidemic
[fx] No. Please try again.
[f*] Excellent!
[l]lung
[fx] No, that’s not correct. Please try again.
[f*] Good!
[l]rare
[fx] No, that’s not correct. Please try again.
[f*] Great!
[l]tobacco
[fx] No, that’s not correct. Please try again.
[f*] Excellent!
[q labels = “top”]
By the 1920s, the increasing rate of lung cancer began to be linked with __________, but other possible causes for increased lung cancer rates were also considered. These included exposure to poison gas suffered by soldiers during World War One and exposure to the tar that was increasingly used on roads as driving became more common.
In the 1930s, population studies in German hospitals led to the discovery that lung cancer patients were far ______________ to have smoked than patients who didn’t have cancer. By the 1950s, American doctors were able to calculate that “smokers of 35 cigarettes per day increased their odds of ________ from lung cancer by a factor of 40.” (tobaccocontrol.bmj.com).
So, by that point there was a clear _______________: if someone smokes, then they have a higher chance of developing lung cancer. Now let’s look at how experiments could be designed to confirm that hypothesis.
[l]dying
[fx] No. Please try again.
[f*] Great!
[l]hypothesis
[fx] No, that’s not correct. Please try again.
[f*] Good!
[l]more likely
[fx] No, that’s not correct. Please try again.
[f*] Great!
[l]smoking
[fx] No, that’s not correct. Please try again.
[f*] Good!
[/qwiz]
3. Controlled Experiments: General Features
For the sake of simplicity (and learning), the experiment described below is somewhat different from the actual animal experiments that were performed to help establish the link between tobacco smoke and cancer.
Let’s start by reviewing what an experiment is: it’s a controlled form of observation that lets you observe one thing at a time. As you read what follows, refer to the diagram below.
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[h] Quiz: Controlled Experiments, General Features
[q multiple_choice=”true”] Experiments try to test the effect of ONE thing at a time. The thing that you test is called the independent variable.
In relationship to our hypothesis (if someone smokes, they have a higher chance of developing lung cancer), what’s the independent variable?
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[c]IHRvYmFjY2 8gc21va2U=[Qq]
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[q] Because we’re testing a harmful substance, we’re not going to test humans, but animals related to humans (like mice or rats). Keep that in mind when we talk about “groups” and “individuals” below.
- One group is the control group. The control group is
- the standard for comparison.
- not exposed to the independent variable.
- The second group is the experimental group. This group gets exposed to the independent variable.
[q multiple_choice=”true”] In relationship to our hypothesis about smoking and cancer, what will be our control group?
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[q multiple_choice=”true”] What’s the experimental group?
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[q multiple_choice=”true”] An experiment is going to have some observable outcome. That outcome is called the dependent variable. In relationship to our hypothesis (if someone smokes, they have a higher chance of developing lung cancer), what’s the dependent variable?
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4. Scientific Method and Experimental Design Fill-in-the-Blanks Quiz
To make sure you understand the key terms we’ve used in this lesson, work through these questions.
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[h]Quiz: The Scientific Method and Controlled Experiments
[q] The step in the scientific method that involves sensing the world, and noticing patterns and relationships is [hangman].
[c]T2JzZXJ2YXRpb24=[Qq]
[q]An educated guess that includes a prediction is a [hangman].
[c]aHlwb3RoZXNpcw==[Qq]
[q]A structured form of observation that allows you to examine one thing at a time is a(n) [hangman].
[c]ZXhwZXJpbWVudA==[Qq]
[q]An experiment tests the validity (or correctness) of a(n) [hangman].
[c]aHlwb3RoZXNpcw==[Qq]
[q]A well-formulated hypothesis includes a [hangman].
[c]cHJlZGljdGlvbg==[Qq]
[q]The thing you test in an experiment is the [hangman] variable.
[c]IGluZGVwZW5kZW50[Qq]
[q]The measured or observed result of the independent variable is the [hangman] variable.
[c]ZGVwZW5kZW50[Qq]
[x]
If you want more practice, please press the button below.
[restart]
[/qwiz]
5. A controlled experiment to test the smoking/cancer connection
So, how would this work in the case of an animal experiment to test the hypothesis that tobacco smoke causes cancer?
Well, if it’s an animal experiment, we need an animal.
Like humans, rats are mammals. Their internal organs, including their lungs, look very much like miniature versions of those in humans. In terms of body chemistry, they’re also very much like us: many of the chemical reactions occurring in our cells are identical. So the reasoning (which is widely accepted in biology) is that if something causes cancer in a rat, it is likely to cause cancer in a human being.
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[h]Using the Scientific method
[q]So here’s our experiment. We’re going to have two rats. In one group, we’ll have a rat that smokes. In the second group, we’ll have a rat that’s exposed to exactly the same conditions (the same food, temperature, etc.). The only difference is that the second rat won’t smoke.
| A non-smoking rat | A smoking rat |
[q]In addition to the difficulty involved in training rats to smoke, what’s the problem with the experiment shown below?
| A non-smoking rat | A smoking rat |
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[q]So, here’s what we’re going to do: We’re going to use two groups of rats, each with enough individuals to show the effect of the independent variable. One group will be exposed to tobacco smoke (which is much easier than training rats to smoke). A second group will be kept under identical conditions, except for the fact that it won’t be exposed to tobacco smoke. We’re going to measure the rate of cancer in each group and see if there’s a difference.
If there is a difference, we can be pretty sure that it’s a result of the presence of the independent variable (tobacco smoke).
[q]What’s the control group in this experiment?
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[q]
What’s the experimental group in this experiment?
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[q]
What’s the dependent variable in this experiment?
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[q]What’s the independent variable in this experiment?
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[x]
[/qwiz]
6. What Happened Next
Animal studies similar in structure to the one described above confirmed the hypothesis that animals exposed to tobacco smoke will have higher rates of cancer. In fact, this had been known since the 1930s, when controlled experiments showed that elements of tobacco smoke, when put into liquid form, could cause tumors to form on the skin of rabbits.
Throughout the 1900s, several lines of evidence confirmed the tobacco/cancer link. These lines of evidence included
- Chemical analysis of tobacco to identify cancer-causing agents,
- Studies of how cells in lung tissue were affected by smoking, and
- Public health studies showing that people who smoked were more likely to develop lung cancer.
However, through much of the 1900s, smoking continued to increase among many populations around the world. This was largely caused by tobacco companies, which continued to market cigarettes and devoted significant amounts of resources to denying the scientific evidence about the dangers of smoking.
A major landmark in turning the tide toward public understanding of the dangers of cigarettes was in 1964. In that year, the U.S. Surgeon General’s office published a report that smoking causes lung cancer, cancer of the larynx, and chronic bronchitis. In 1966, the U.S. Federal Government mandated that warning labels be placed on cigarette packages. In the following years, cigarette smoking in the US declined from about 45% (in 1965) to about 12.5% today.
Cigarette manufacturers have responded to the decline by marketing their products overseas. In 2020, 22% of the Earth’s population regularly used tobacco. That’s over 1 billion people. The related global death toll is estimated to be eight million deaths/year.
You can read the entire story by following the links at the bottom of this tutorial.
7. Advanced Topics
7.1. Null v. Alternative Hypothesis
How do you know if there’s a relationship between the independent variable and the experimental results? Let’s say that you are testing the effects of tobacco tar placed on the skin of rabbits. You have 50 rabbits in the control group (not exposed to the tobacco tar), and 50 in the experimental group (exposed to the tobacco tar). What if your experiment results in 1 rabbit in the control group developing cancer, and 2 rabbits in the experimental group developing cancer? Is this a significant difference? What if 2 rabbits develop cancer in the control group, and 6 rabbits develop cancer in the experimental group? What difference matters?
We’re now in the realm of statistics, which is beyond the scope of this tutorial (Note: you can learn about standard error and statistically significant differences through this handout). But to understand what’s below, just accept the fact that statisticians have developed tests to see if there’s a statistically significant difference between the results in the control group and the experimental group. What we’re interested in below is how they set up these tests.
To determine significance, you’re actually comparing two hypotheses.
- The null hypothesis is the claim that there’s no statistically significant relationship between the independent and the dependent variable. In other words, in an experiment designed to examine the relationship between exposure to cigarette smoke and cancer, the null hypothesis would be that there’s no relationship between exposure to cigarette smoke and the incidence of cancer.
- The alternative hypothesis claims that the null hypothesis is incorrect and that there is a statistically significant relationship between the independent and the dependent variable.
[qdeck bold_text=”false” style=”width: 550px !important; min-height: 400px !important;” qrecord_id=”sciencemusicvideosMeister1961-Null v. Alternative Hypoth (2.0)”]
[h] Null vs. alternative hypothesis practice
[q] A researcher wants to see if a certain diet reduces the incidence of heart disease. What is the null hypothesis? The alternative hypothesis?
[a] The null hypothesis is that in a controlled study, there will be no statistically significant difference between people following the diet (the experimental group) and people not following the diet (the control group)
The alternative hypothesis is that there will be a statistically significant reduction in heart disease in people following the diet (the experimental group) compared to people not following the diet (the control group).
[q] A biologist claims that the dark fur on populations of mice living on dark lava flows evolved as an adaptation for camouflage, protecting the mice from aerial predators like hawks and owls. To test this claim, she releases takes 50 mice and divides them into two groups. 25 mice are placed in an enclosure with dark rock. 25 are placed in an adjacent enclosure with light-colored rock. Both enclosures are open from above.
What is the null hypothesis? The alternative hypothesis?
[a] The null hypothesis is that in there will be no statistically significant difference between the amount of predation suffered by the mice placed in the light-colored rock enclosure (the experimental group) and the mice placed in the dark-rock enclosure (the control group).
The alternative hypothesis is that there will be a statistically significant reduction in predation in the light-colored rock enclosure (the experimental group) compared to the mice in the dark-colored rock enclosure (the control group).
[/qdeck]
7.2. Positive v. Negative Controls
In the smoking-causes-cancer experiments described above, the independent variable was tobacco smoke (or tar). In the control group, this independent variable was absent. That’s an example of an experiment with a negative control group. A negative control is one in which the control group is exposed to nothing (no independent variable) or to a placebo.
A placebo is a substance or treatment which is known to have no effect. A typical placebo is a sugar pill, which would be administered to individuals in the control group at the same time as a pill with an experimental treatment would be administered to the subjects in an experimental group.
Why use a placebo? The original idea, which goes back centuries, is that the act of taking any medicine can (at least temporarily) alleviate the symptoms of some diseases. This is particularly true of conditions that involve pain or discomfort. So, to cancel out this effect, studies to test the effectiveness of medicines typically use a placebo for comparison.
A positive control is one in which a new treatment is compared to another treatment with a known effect.
Here’s an example. A student wants to test the effect of blue light on plant growth. His hypothesis is that blue light because it has more energy than white light, will increase plant growth. Plants grown under blue light will be the experimental group. But what will serve as a control group? A negative control could be plants grown under no light at all. But our student knows that plants need light to grow. So his design will involve a positive control: plants grown under normal light (controlled to be the same intensity as the blue light used in the experimental group.
Here’s another example.
[qwiz]
[h] Understanding experimental controls
[q]A company is testing a new anti-fungal cream to fight athlete’s foot. They find a suitably large group of athlete’s foot sufferers upon which to test the cream. They divide the group into three. See if you can figure out which group is the experimental group, which group is the negative control, and which group is the positive control.
- Group A receives the new antifungal cream and applies it based on a set regimen.
- Group B receives a placebo: the cream has an identical smell and feel, but no active ingredient.
- Group C receives a cream from a competitor that’s known to have a certain level of effectiveness.
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[/qwiz]
Try the flashcards below.
[qdeck bold_text=”false” style=”width: 550px !important; min-height: 400px !important;” qrecord_id=”sciencemusicvideosMeister1961-Pos v Neg Controls (2.0)”]
[h] Negative vs Positive Controls Practice
[q] A researcher wants to see if a newly designed knee brace reduces discomfort in runners who are experiencing knee pain. What would be a negative control in this experiment? A positive control?
[a]In this experiment, a negative control would compare a group of runners using the new knee brace (experimental group) with ones who don’t use a knee brace (control group).
A positive control would compare a group of runners using the new know brace (experimental group) with ones using another knee brace that’s already known to relieve knee pain (control group).
[q]A public health officer is concerned that kale from a certain farm might be causing bacterial infections. To test the presence of bacteria on something, a typical test is to touch it with a cotton swab, then rub that swab on a sterile petri dish with a medium upon which bacteria can grow. How would the public health officer provide a negative and a positive control test for the kale? Why do they need a positive control?
[a]The negative control design would involve swabbing the kale, then rubbing the swab on a sterile Petri dish to create an experimental group. The negative control group would involve comparing that Petri dish to a second sterile Petri dish that was not exposed to any source of bacteria at all.
A positive control design would compare the Petri dish that had been swabbed from the kale to a second Petri dish that was swabbed with a known source of bacteria. The positive control shows that these Petri dishes, when inoculated with bacteria, will support bacterial growth.
[/qdeck]
7. Checking Understanding Quiz
In this tutorial, we’ve learned about
- The steps of the scientific method
- Observe
- Hypothesize
- Experiment
- Draw conclusions
- The design of a controlled experiment
- independent variable
- dependent variable
- control group
- experimental group
- Additional features of a well-designed experiment
- It tests only one thing (the independent variable)
- It uses large enough groups to avoid random results based on individual differences.
- Null and Alternative hypotheses
- Positive and Negative Controls
To make sure you’ve mastered this material, take the quiz below.
[qwiz style = “border: 3px solid black; ” qrecord_id=”sciencemusicvideosMeister1961-Sci. Method and Exper Design Quiz (2.0)”]
[h]Quiz: The Scientific Method and Designing Experiments
[i] Here’s how the quiz works:
- Each question is multiple-choice, but the entire quiz is like a series of flashcards.
- If you get the question right, it comes off the deck.
- If you get the question wrong, it goes to the bottom of the deck, so you can try it again.
[q] Noticing patterns in the world around you is best classified as
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[q] An educated guess that includes a prediction is a(n)
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[q] A structured form of observation that allows you to observe one thing at a time is a(n)
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[q] This includes a prediction, and is best put in an “if…then…” format
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[q] This part of the scientific method involves testing whether a hypothesis is correct.
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[q]In an experiment, the thing you’re testing is the
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[q]In an experiment, the measured result is the
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[q]The part of the experiment that serves as a standard for comparison, and which shows you what the result would be without the independent variable, is the
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[q]Two students have designed an experiment to test the effect of loud bass notes on reproduction rates in guppies (a small aquarium fish). They divide the guppies into two groups of 15, each group in its own 20-gallon aquarium tank. One tank is exposed to the loud bass notes, and one is not.
In this experiment, which of the following is the independent variable?
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[f]WWVzLiBUaGUgaW5kZXBlbmRlbnQgdmFyaWFibGUgaXMgdGhlIHRoaW5nIHlvdSYjODIxNztyZSB0ZXN0aW5nLiBXaGF0JiM4MjE3O3MgYmVpbmcgdGVzdGVkIGhlcmUgaXMgdGhlIGxvdy1mcmVxdWVuY3kgc291bmQu
[f]Tm8uIFRoZSBpbmRlcGVuZGVudCB2YXJpYWJsZSBpcyB0aGUgdGhpbmcgeW91JiM4MjE3O3JlIHRlc3RpbmcuIFdoYXQmIzgyMTc7cyBiZWluZyB0ZXN0ZWQ/
Cg==[Qq]
[q]Two students have designed an experiment to test the effect of low bass notes on reproduction rates in guppies (a small aquarium fish). They divide the guppies into two groups of 15, each group in its own 20-gallon aquarium tank. One tank is exposed to the low bass notes, and one is not.
In this experiment, which of the following is the dependent variable?
[c]dGhlIGd1cHBpZXMgZXhwb3NlZCB0byB0aGUgbG93wqBiYXNzIG5vdGVz[Qq]
[c]dGhlIGd1cHBpZXMgTk9UwqBleHBvc2VkIHRvIHRoZSBsb3fCoGJhc3Mgbm90ZXM=[Qq]
[c]dGhlIGxvd8KgYmFzcyBub3Rlcw==[Qq]
[c]dGhlIHJhdGUgb2Yg cmVwcm9kdWN0aW9u[Qq]
[f]Tm8uwqBUaGUgZGVwZW5kZW50IHZhcmlhYmxlIGlzIHRoZSByZXN1bHQgeW91IGdldC4gVGhlc2UgZXhwZXJpbWVudGVycyB3YW50IHRvIHNlZSBpZiBsb3cgYmFzcyBub3Rlc8KgYWZmZWN0IHNvbWV0aGluZy4gV2hhdCBpcyB0aGF0IHNvbWV0aGluZz8=[Qq]
[f]Tm8uwqBUaGUgZGVwZW5kZW50IHZhcmlhYmxlIGlzIHRoZSByZXN1bHQgeW91IGdldC4gVGhlc2UgZXhwZXJpbWVudGVycyB3YW50IHRvIHNlZSBpZiBsb3cgYmFzcyBub3Rlc8KgYWZmZWN0IHNvbWV0aGluZy4gV2hhdCBpcyB0aGF0IHNvbWV0aGluZz8=[Qq]
[f]Tm8uwqBUaGUgZGVwZW5kZW50IHZhcmlhYmxlIGlzIHRoZSByZXN1bHQgeW91IGdldC4gVGhlc2UgZXhwZXJpbWVudGVycyB3YW50IHRvIHNlZSBpZiBsb3cgYmFzZSBub3RlcyBhZmZlY3Qgc29tZXRoaW5nLiBXaGF0IGlzIHRoYXQgc29tZXRoaW5nPw==
[f]WWVzLsKgVGhlIGRlcGVuZGVudCB2YXJpYWJsZSBpcyB0aGUgcmVzdWx0IHlvdSBnZXQuIFRoZXNlIGV4cGVyaW1lbnRlcnMgd2FudCB0byBzZWUgaWYgbG93IGJhc3PCoG5vdGVzIGFmZmVjdCByZXByb2R1Y3Rpb24gcmF0ZXMuIFRoZXJlZm9yZSByZXByb2R1Y3Rpb24gcmF0ZXMgYXJlIHRoZSBpbmRlcGVuZGVudCB2YXJpYWJsZS4=
Cg==[Qq]
[q]Two students have designed an experiment to test the effect of low bass notes on reproduction rates in guppies (a small aquarium fish). They divide the guppies into two groups of 15, each group in its own 20-gallon aquarium tank. One tank is exposed to the low bass notes, and one is not.
In this experiment, which of the following is the control group?
[c]dGhlIGd1cHBpZXMgZXhwb3NlZCB0byB0aGUgbG93IGJhc3Mgbm90ZXM=[Qq]
[c]dGhlIGd1cHBpZXMgTk9UwqBleHBvc2Vk IHRvIHRoZSBsb3cgYmFzcyBub3Rlcw==[Qq]
[c]dGhlIGxvdyBiYXNzIG5vdGVz[Qq]
[c]dGhlIHJhdGUgb2YgcmVwcm9kdWN0aW9u[Qq]
[f]Tm8uwqBUaGXCoGNvbnRyb2wgZ3JvdXAgaXMgdGhlIGdyb3VwIHRoYXQgaXMgbm90IGV4cG9zZWQgdG8gdGhlIGluZGVwZW5kZW50IHZhcmlhYmxlLsKgSnVzdCBmaWd1cmUgb3V0IHdoYXQgdGhlIGluZGVwZW5kZW50IHZhcmlhYmxlIGlzIChpdCYjODIxNztzIHdoYXQgeW91JiM4MjE3O3JlIHRlc3RpbmcpLCBhbmQgeW91JiM4MjE3O2xsIGhhdmUgeW91ciBhbnN3ZXIu[Qq]
[f]TmljZS7CoFRoZcKgY29udHJvbCBncm91cCBpcyB0aGUgZ3JvdXAgdGhhdCBpcyA=bm90IGV4cG9zZWQgdG8gdGhlIGluZGVwZW5kZW50IHZhcmlhYmxlLsKgSW4gdGhpcyBleHBlcmltZW50LCBpdCYjODIxNztzIHRoZSBncm91cCBub3QgZXhwb3NlZCB0byBsb3fCoGJhc3Mgbm90ZXM=[Qq]
[f]Tm8uwqBUaGXCoGNvbnRyb2wgZ3JvdXAgaXMgdGhlIGdyb3VwIHRoYXQgaXMgbm90IGV4cG9zZWQgdG8gdGhlIGluZGVwZW5kZW50IHZhcmlhYmxlLsKgSnVzdCBmaWd1cmUgb3V0IHdoYXQgdGhlIGluZGVwZW5kZW50IHZhcmlhYmxlIGlzIChpdCYjODIxNztzIHdoYXQgeW91JiM4MjE3O3JlIHRlc3RpbmcpLCBhbmQgeW91JiM4MjE3O2xsIGhhdmUgeW91ciBhbnN3ZXIu
[f]Tm8uwqBUaGXCoGNvbnRyb2wgZ3JvdXAgaXMgdGhlIGdyb3VwIHRoYXQgaXMgbm90IGV4cG9zZWQgdG8gdGhlIGluZGVwZW5kZW50IHZhcmlhYmxlLsKgSnVzdCBmaWd1cmUgb3V0IHdoYXQgdGhlIGluZGVwZW5kZW50IHZhcmlhYmxlIGlzIChpdCYjODIxNztzIHdoYXQgeW91JiM4MjE3O3JlIHRlc3RpbmcpLCBhbmQgeW91JiM4MjE3O2xsIGhhdmUgeW91ciBhbnN3ZXIu
Cg==[Qq]
[q]Two students have designed an experiment to test the effect of low bass notes on reproduction rates in guppies (a small aquarium fish). They divide the guppies into two groups of 15, each group in its own 20-gallon aquarium tank. One tank is exposed to the low bass notes, and one is not.
In this experiment, which of the following is the experimental group?
[c]dGhlIGd1cHBpZXMgZXhwb3NlZCB0 byB0aGUgbG93IGJhc3Mgbm90ZXM=[Qq]
[c]dGhlIGd1cHBpZXMgTk9UwqBleHBvc2VkIHRvIHRoZSBsb3fCoGJhc3Mgbm90ZXM=[Qq]
[c]dGhlIGxvd8KgYmFzcyBub3Rlcw==[Qq]
[c]dGhlIHJhdGUgb2YgcmVwcm9kdWN0aW9u[Qq]
[f]WWVzLiBUaGUgZXhwZXJpbWVudGFswqBncm91cCBpcyB0aGUgZ3JvdXAgdGhhdCA=aXPCoGV4cG9zZWQgdG8gdGhlIGluZGVwZW5kZW50IHZhcmlhYmxlLsKgSW4gdGhpcyBleHBlcmltZW50LCB0aGUgaW5kZXBlbmRlbnQgdmFyaWFibGUgaXMgdGhlIGxvd8KgYmFzcyBub3RlcywgYW5kIHRoZSBndXBwaWVzIGV4cG9zZWQgdG8gdGhpcyBzb3VuZCBtYWtlIHVwIHRoZSBleHBlcmltZW50YWwgZ3JvdXAu[Qq]
[f]Tm8uIFRoZSBleHBlcmltZW50YWzCoGdyb3VwIGlzIHRoZSBncm91cCB0aGF0IA==aXMgZXhwb3NlZCB0byB0aGUgaW5kZXBlbmRlbnQgdmFyaWFibGUuIEp1c3QgZmlndXJlIG91dCB3aGF0IHRoZSBpbmRlcGVuZGVudCB2YXJpYWJsZSBpcyAoaXQmIzgyMTc7cyB3aGF0IHlvdSYjODIxNztyZSB0ZXN0aW5nKSwgYW5kIHlvdSYjODIxNztsbCBoYXZlIHlvdXIgYW5zd2VyLsKg[Qq]
[f]Tm8uIFRoZSBleHBlcmltZW50YWzCoGdyb3VwIGlzIHRoZSBncm91cCB0aGF0IA==aXMgZXhwb3NlZCB0byB0aGUgaW5kZXBlbmRlbnQgdmFyaWFibGUuIEp1c3QgZmlndXJlIG91dCB3aGF0IHRoZSBpbmRlcGVuZGVudCB2YXJpYWJsZSBpcyAoaXQmIzgyMTc7cyB3aGF0IHlvdSYjODIxNztyZSB0ZXN0aW5nKSwgYW5kIHlvdSYjODIxNztsbCBoYXZlIHlvdXIgYW5zd2VyLg==
[f]Tm8uIFRoZSBleHBlcmltZW50YWzCoGdyb3VwIGlzIHRoZSBncm91cCB0aGF0IA==aXMgZXhwb3NlZCB0byB0aGUgaW5kZXBlbmRlbnQgdmFyaWFibGUuIEp1c3QgZmlndXJlIG91dCB3aGF0IHRoZSBpbmRlcGVuZGVudCB2YXJpYWJsZSBpcyAoaXQmIzgyMTc7cyB3aGF0IHlvdSYjODIxNztyZSB0ZXN0aW5nKSwgYW5kIHlvdSYjODIxNztsbCBoYXZlIHlvdXIgYW5zd2VyLg==
Cg==[Qq]
[q]Bob wants to test whether lemon juice can keep dandelion weeds from growing in his garden. He creates several solutions of lemon juice. He then takes dandelion seeds and sprouts them on paper towels. Each day, he sprays the same amount of each solution on the seeds. The data are shown above.
| Solution | Number of Seeds | Percentage of seedlings that sprout |
| 10% lemon juice | 15 | 42% |
| 30% lemon juice | 15 | 44% |
| 50% lemon juice | 15 | 41% |
Based on the data, what’s the problem with the design of Bob’s experiment
[c]SGUgaGFkIG5vIGRlcGVuZGVudCB2YXJpYWJsZQ==[Qq]
[c]SGUgaGFzIG1vcmUgdGhhbiBvbmXCoGluZGVwZW5kZW50IHZhcmlhYmxl[Qq]
[c]SGUgaGFkIG5vIGNv bnRyb2wgZ3JvdXA=[Qq]
[c]dGhlIHNhbXBsZSBzaXplIGlzIHRvbyBzbWFsbA==[Qq]
[f]Tm8uwqBUaGXCoGRlcGVuZGVudCB2YXJpYWJsZSBpcyB0aGUgb3V0Y29tZSwgYW5kIEJvYiYjODIxNztzIGV4cGVyaW1lbnQgaGFzIGEgY2xlYXIgb3V0Y29tZSAodGhlIHBlcmNlbnRhZ2Ugb2Ygc2VlZHMgc3Byb3V0aW5nKQ==[Qq]
[f]Tm8uIEhlJiM4MjE3O3MgdGVzdGluZyB2YXJpb3VzIGNvbmNlbnRyYXRpb25zIG9mIHRoZSBzYW1lIHRoaW5nLCB3aGljaCBpcyBhIGZpbmUgcHJvY2VkdXJlLg==[Qq]
[f]WWVzLsKgVGhlcmUmIzgyMTc7cyBubyBjb250cm9sIGdyb3VwLiBXaXRob3V0IGEgY29udHJvbCBncm91cCwgaXQmIzgyMTc7cyBpbXBvc3NpYmxlIHRvIGtub3cgd2hldGhlciBoYXZpbmcgYWJvdXQgNDAlIG9mIHNlZWRsaW5ncyBzcHJvdXRpbmcgaXMgYSByZXN1bHQgb2YgdGhlIGxlbW9uIGp1aWNlLCBvciB3aGV0aGVyIHRoYXQmIzgyMTc7cyBub3JtYWwgZm9yIGRhbmRlbGlvbiBzZWVkcy4=
[f]Tm8gKGJ1dCB0aGF0JiM4MjE3O3MgYSBzbWFydCByZXNwb25zZSkuIEl0IHRha2VzIGEgbG90IG9mIHN0YXRpc3RpY2FsIGtub3dsZWRnZSB0byBkZXRlcm1pbmUgdGhlIHJpZ2h0IHNhbXBsZSBzaXplLiAxNSBtaWdodCBiZSBva2F5LiBCdXQsIHRoZXJlJiM4MjE3O3MgYSBtdWNoIGJpZ2dlciBwcm9ibGVtIHdpdGggdGhpcyBleHBlcmltZW50Lg==
Cg==[Qq]
[q]Clara is testing whether iron pills will help skinny dogs gain weight. For her experiment, she takes three dogs, a poodle, a boxer, and a collie. She adds iron to their food for two weeks and then records their weight. Here are her results
| Dog Breed | Number of 5-gram iron pills | Weight gain (kilograms) |
| Poodle | 1 | 4 |
| Boxer | 3 | 2 |
| Collie | 5 | 6 |
Based on the data, what’s the problem with the design of her experiment
[c]U2hlIGhhZCBubyBkZXBlbmRlbnQgdmFyaWFibGU=[Qq]
[c]U2hlIGhhcyBtb3JlIHRoYW4gb25lwq BpbmRlcGVuZGVudCB2YXJpYWJsZQ==[Qq]
[c]U2hlIGhhcyBhIGNsZWFyIGJpYXMgdG93YXJkIENvbGxpZXMu[Qq]
[f]Tm8uwqBUaGXCoGRlcGVuZGVudCB2YXJpYWJsZSBpcyB0aGUgb3V0Y29tZSwgYW5kIENsYXJhJiM4MjE3O3PCoGV4cGVyaW1lbnQgaGFzIGEgY2xlYXIgb3V0Y29tZSAod2VpZ2h0IGdhaW4pLiBUaGluayBhYm91dCBob3cgbWFueSB0aGluZ3Mgc2hlJiM4MjE3O3MgdGVzdGluZy4=[Qq]
[f]WWVzLiBTaGUmIzgyMTc7cyB1c2luZyBkb2dzIG9mIGEgZGlmZmVyZW50IGJyZWVkIGFuZCBnaXZpbmcgdGhlbSBhIGRpZmZlcmVudCBudW1iZXIgb2YgcGlsbHMsIHdoaWNoIG1lYW5zIHRoYXQgc2hlJiM4MjE3O3MgdGVzdGluZyBhdCBsZWFzdCB0d28gdGhpbmdzLg==[Qq]
[f]Tm8uIFRoZXJlJiM4MjE3O3Mgbm8gZXZpZGVuY2XCoGZvciBjbGFpbWluZyB0aGF0IHNoZSYjODIxNztzIGJpYXNlZC4gQnV0IHRoaW5rIGFib3V0IGhvdyBtYW55IHRoaW5ncyBzaGUmIzgyMTc7cyB0ZXN0aW5nJiM4MjMwOw==
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8. The Scientific Method Song
For a fun review of the basics of experimental design, watch my music video about the scientific method.
9. Next steps
- If you’re interested in learning a lot more about the history of research about the smoking/cancer connection and the Tobacco industry’s ongoing attempts to obfuscate the science and confuse the public, read The Shameful Past: The history of the discovery of the cigarette-cancer link.“ This is the recommended reading in the student learning guide that goes with this module.
- Use the menu choices above to choose another module. In most AP Bio curricula, you’ll move on to Unit 1, The Chemistry of Life.