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9/21/20 Identify the control group, independent variable, and dependent variable Learn with flashcards, games, and more — for free.
The Scientific Method and Parts of an Experiment. 1. Make an Observation 2. Develop a Hypothesis 3. Make an Experiment 4. Carry out the Experiment and Analyze results 5. Draw Conclusions.
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- Overview
- Introduction
- How are hypotheses tested?
- Controlled experiments
- Control and experimental groups
- Independent and dependent variables
- Variability and repetition
- Controlled experiment case study: CO2 and coral bleaching
- Non-experimental hypothesis tests
- Case study: Coral bleaching and temperature
How scientists conduct experiments and make observations to test hypotheses.
•One pot of seeds gets watered every afternoon.
•The other pot of seeds doesn't get any water at all.
After a week, nine out of ten seeds in the watered pot have sprouted, while none of the seeds in the dry pot have sprouted. It looks like the "seeds need water" hypothesis is probably correct!
Biologists and other scientists use the scientific method to ask questions about the natural world. The scientific method begins with an observation, which leads the scientist to ask a question. They then come up with a hypothesis, a testable explanation that addresses the question.
A hypothesis isn't necessarily right. Instead, it's a "best guess," and the scientist must test it to see if it's actually correct. Scientists test hypotheses by making predictions: if hypothesis X is right, then Y should be true. Then, they do experiments or make observations to see if the predictions are correct. If they are, the hypothesis is supported. If they aren't, it may be time for a new hypothesis.
When possible, scientists test their hypotheses using controlled experiments. A controlled experiment is a scientific test done under controlled conditions, meaning that just one (or a few) factors are changed at a time, while all others are kept constant. We'll look closely at controlled experiments in the next section.
In some cases, there is no good way to test a hypothesis using a controlled experiment (for practical or ethical reasons). In that case, a scientist may test a hypothesis by making predictions about patterns that should be seen in nature if the hypothesis is correct. Then, they can collect data to see if the pattern is actually there.
What are the key ingredients of a controlled experiment? To illustrate, let's consider a simple (even silly) example.
Suppose I decide to grow bean sprouts in my kitchen, near the window. I put bean seeds in a pot with soil, set them on the windowsill, and wait for them to sprout. However, after several weeks, I have no sprouts. Why not? Well...it turns out I forgot to water the seeds. So, I hypothesize that they didn't sprout due to lack of water.
To test my hypothesis, I do a controlled experiment. In this experiment, I set up two identical pots. Both contain ten bean seeds planted in the same type of soil, and both are placed in the same window. In fact, there is only one thing that I do differently to the two pots:
•One pot of seeds gets watered every afternoon.
•The other pot of seeds doesn't get any water at all.
After a week, nine out of ten seeds in the watered pot have sprouted, while none of the seeds in the dry pot have sprouted. It looks like the "seeds need water" hypothesis is probably correct!
There are two groups in the experiment, and they are identical except that one receives a treatment (water) while the other does not. The group that receives the treatment in an experiment (here, the watered pot) is called the experimental group, while the group that does not receive the treatment (here, the dry pot) is called the control group. The control group provides a baseline that lets us see if the treatment has an effect.
[Is there always one experimental group and one control group?]
The factor that is different between the control and experimental groups (in this case, the amount of water) is known as the independent variable. This variable is independent because it does not depend on what happens in the experiment. Instead, it is something that the experimenter applies or chooses him/herself.
[Is there always one independent variable and one dependent variable?]
In contrast, the dependent variable in an experiment is the response that's measured to see if the treatment had an effect. In this case, the fraction of bean seeds that sprouted is the dependent variable. The dependent variable (fraction of seeds sprouting) depends on the independent variable (the amount of water), and not vice versa.
Experimental data (singular: datum) are observations made during the experiment. In this case, the data we collected were the number of bean sprouts in each pot after a week.
Out of the ten watered bean seeds, only nine came up. What happened to the tenth seed? That seed may have been dead, unhealthy, or just slow to sprout. Especially in biology (which studies complex, living things), there is often variation in the material used for an experiment – here, the bean seeds – that the experimenter cannot see.
Because of this potential for variation, biology experiments need to have a large sample size and, ideally, be repeated several times. Sample size refers to the number of individual items tested in an experiment – in this case, 10 bean seeds per group. Having more samples and repeating the experiment more times makes it less likely that we will reach a wrong conclusion because of random variation.
As a more realistic example of a controlled experiment, let's examine a recent study on coral bleaching. Corals normally have tiny photosynthetic organisms living inside of them, and bleaching happens when they leave the coral, typically due to environmental stress. The photo below shows a bleached coral in front and a healthy coral in back.
A lot of research on the cause of bleaching has focused on water temperature1 . However, a team of Australian researchers hypothesized that other factors might be important too. Specifically, they tested the hypothesis that high CO2 levels, which make ocean waters more acidic, might also promote bleaching2 .
What kind of experiment would you do to test this hypothesis? Think about:
•What your control and experimental groups would be
•What your independent and dependent variables would be
•What results you would predict in each group
Some types of hypotheses can't be tested in controlled experiments for ethical or practical reasons. For example, a hypothesis about viral infection can't be tested by dividing healthy people into two groups and infecting one group: infecting healthy people would not be safe or ethical. Similarly, an ecologist studying the effects of rainfall can't make it rain in one part of a continent, while keeping another part dry as a control.
In situations like these, biologists may use non-experimental forms of hypothesis testing. In a non-experimental hypothesis test, a researcher predicts observations or patterns that should be seen in nature if the hypothesis is correct. They then collect and analyze data, seeing whether the patterns are actually present.
A good example of hypothesis testing based on observation comes from early studies of coral bleaching. As mentioned above, bleaching is when corals lose the photosynthetic microorganisms that live inside of them, which makes them turn white. Researchers suspected that high water temperature might cause bleaching, and they tested this hypothesis experimentally on a small scale (using isolated coral fragments in tanks)3,4 .
What ecologists most wanted to know, however, was whether water temperature was causing bleaching for lots of different coral species in their natural setting. This broader question could not be answered experimentally, as it wouldn't be ethical (or even possible) to artificially change the water temperature surrounding entire coral reefs.
Instead, to test the hypothesis that natural bleaching events were caused by increases in water temperature, a team of researchers wrote a computer program to predict bleaching events based on real-time water temperature data. For example, this program would generally predict bleaching for a particular reef when the water temperature in the reef's area exceeded its average monthly maximum by 1 ∘C or more1 .
The computer program was able to predict many bleaching events weeks or even months before they were reported, including a large bleaching event in the Great Barrier Reef in 19981 . The fact that a temperature-based model could predict bleaching events supported the hypothesis that high water temperature causes bleaching in naturally occurring coral reefs.
Scientific method example: Failure to toast. Let's build some intuition for the scientific method by applying its steps to a practical problem from everyday life. 1. Make an observation. Let's suppose that you get two slices of bread, put them into the toaster, and press the button. However, your bread does not toast. 2. Ask a question.
- Well, you could consider it to be both. If you say, "If I do X then Y will happen" you are predicting what will happen if you do something.
- Theories don't become laws. Here is a good resource for defining how these scientific concepts relate: http://ncse.com/evolution/education/definiti...
- You can never be 100% sure that something is _right_. You can just be sure that you've followed the Scientific Method, and that your results fits s...
- Exactly, like others said if you do more experiments, then you will be sure to find out which info is credible. When I did experiments, my teacher...
- I think that there is no definite format to writing a hypothesis, but as you said, yes there is a 'proper' and appropriate way to writing a hypothe...
- Why do you think you get stuck at conclusions? Are you afraid to say what you mean? Have you ever taken Logic classes? I haven't but after quite a...
- You can't be expected to come up with an accurate hypothesis every time, this is why you have to do observations and/or experiments to test the hyp...
- A hypothesis does not always have to be correct. The scientific method aims to prove or disprove the hypothesis and continue from there.
The scientific method is a logical approach to understanding the world. It starts with an observation, followed by a question. A testable explanation or hypothesis is then created. An experiment is designed to test the hypothesis, and based on the results, the hypothesis is refined.
- 12 min
- Sal Khan
People also ask
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Apr 28, 2017 · A controlled experiment is a scientific test that is directly manipulated by a scientist, in order to test a single variable at a time. The variable being tested is the independent variable, and is adjusted to see the effects on the system being studied.
