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Observations and Experiments

Observations and experiments are used to test hypotheses.

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Observations and Experiments

How do you test a hypothesis?

When you test a hypothesis, you must make observations or perform experiments.


  • experiment: A trial made under controlled conditions to test the validity of a hypothesis.
  • independent variable (IV): The variable in an experiment that is controlled and changed by the researcher. Think of this as the possible "cause", in the cause-and-effect relationship, that is being tested.
  • dependent variable (DV): The variable in an experiment that is being measured to see how it changes when the independent variable is changed. Think of this as the possible "effect", in the cause-and-effect relationship, that is being tested.
  • controlled conditions (CC): All of the variables of an experiment that are kept the same between the control group and the experimental group.  If only one thing, the IV, is changed then it helps us know why the DV is different between the two versions of the experiment.
  • control group: A version of a scientific experiment in which the factor being tested, the independent variable, IS NOT changed; The dependent variable of the control group is then compared to the dependent variable of the experimental group.
  • experimental group: A version of a scientific experiment in which the factor being tested, the independent variable, IS changed; We compare the resulting dependent variable with the dependent variable from the control group.  Comparing the DV from the control group to the DV of the experimental group helps us determine causation.

Testing Hypotheses

If we were doing a scientific investigation we need to gather the information to test the hypotheses ourselves. We would do this by making observations or running experiments.  The purpose is to try to determine if there exists a cause-and-effect relationship between the IV and the DV.


Observations of Earth's surface may be made from the land surface or from space. Many important observations are made by orbiting satellites, which have a bird's eye view of how the planet is changing.

This satellite image shows how the extent of glaciers in Glacier National Park has changed in recent years.

Often, observation is used to collect data when it is not possible for practical or ethical reasons to perform experiments. Scientists may send devices to make observations for them when it is too dangerous or impractical for them to make the observations directly. They may use microscopes to explore tiny objects or telescopes to learn about the universe.

Artist's concept of the Juno orbiter circling Jupiter. The mission is ongoing.


Answering some questions requires experiments. An experiment is a test that may be performed in the field or in a laboratory. An experiment must always done under controlled conditions. The goal of an experiment is to verify or falsify a hypothesis.

In an experiment, it is important to change only one factor. All other factors must be kept the same.

  • Independent variable: The factor that will be manipulated.
  • Dependent variable: The factors that depend on the independent variable.

An experiment must have a control group.  For example, if you want to test if Vitamin C prevents colds, you must divide your test subjects up so that some people receive Vitamin C and some do not. Those who do not receive the Vitamin C are the control group.  The other people will be given Vitamin C.  They are known as the experimental group. We want to know if Vitamin C prevents colds, therefore everything between the experimental and the control groups is the same, except for taking Vitamin C.  This is very difficult to do - the age, sex, health, diet, exercise, exposure to germs, etc. must be exactly the same for all of the people in each group.  This way if people in the experimental group do not catch colds (and the people of the control group do), then it is obvious that the Vitamin C taken by the experimental group prevented them from catching a cold, because every other condition was exactly same as the control group.

A Non-example.

It can be easier to understand the reason for a controlled experiment if we consider a non-example, or an experiment without controlled conditions.  Let's say we would like to know what happens to a plant if we use fertilizer.  Our IV is the amount of fertilizer and the DV is the height of the flowers. In or our control group, we use 10 tulip plants and in our experimental group we use 10 tulip plants.   In our control group we put all ten plants in a sunny window, water them every morning, and add NO fertilizer.  In our experimental group, we add Miracle-Gro fertilizer to each pot, place them in the shade under a bench and water them every other day. 

After 15 days we measure the flowers of each group, and we are shocked to see that the flowers of the experimental group (fertilizer added) are shorter than the flowers of the control group (no fertilizer).  However, there is no way we can tell the mechanism of causation.  Was it the fertilizer?  Was it the amount of sun?  Was it the amount of water?  Too many variables were different between the control group and the experimental group for us to know what caused the flowers to grow different heights.


  • Testing a hypothesis requires data. Data can be gathered by observations or by experiments.
  • Observations can be done simply by looking at and measuring a phenomenon, or by using advanced technology.
  • Experiments must be well-designed. They must be done under controlled conditions and with the manipulation of only one variable.


Use these resources to answer the questions that follow.


1. What is an observation?

2. What is an inference?

3. For each slideshow picture, list an observation. Now, for each picture list an inference. How can you tell the difference?

Go to the following website: http://www.sciencekids.co.nz/gamesactivities/rockssoils.html

Conduct the experiments to find the following answers:

4. Which rock floats?

5. Which rocks are permeable?

6. Which rocks can be split?

7. Which rocks wear well?

8. How did you do experiments to find the answers to the questions?


1. Under what circumstances would a scientist test a hypothesis using observations?

2. Under what circumstances would a scientist test a hypothesis using experiments?

3. What is the difference between an independent and a dependent variable in an experiment?

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