Scientific Method

1. Description of the Scientific Method

One of the goals of science is to come up with explanations about how the natural world (all the things we see or experience) functions. Although there are other systems for understanding and explaining the world around us (such as religion and traditional beliefs) science differs from these in that scientific explanations are based on laws of nature. Laws of nature are patterns in nature that are objective (do not depend on faith, authority, or opinion), are testable (can be demonstrated with experiments), and are consistent (the same conditions produce the same results). 

The 4 Steps of the Scientific Method

To learn about the natural world, scientists use a four step procedure called the scientific method. The four steps of the scientific method are listed below. To help illustrate the scientific method, an example that an entomologist (a biologist who specializes in insects) might use is given in italics below each step.

Step 1: Observations & Questions

Observe something in the natural world and ask a question about how it works. The part of the natural world that is observed and investigated is usually the area that the scientist specializes in. An entomologist for example, would ask questions about how insects function.

“The life cycle of a fruit fly is about 30 days (at 29 degrees Celsius). How do changes in temperature affect the life cycle of a fruit fly?” 

Step 2: Hypothesis

Make a hypothesis (an educated guess) which attempts to answer the question. A useful hypothesis is a testable statement.

“Decreasing the temperature of a fruit fly's environment will increase the time it takes the fruit fly to complete its life cycle.”

Step 3: Experiment

Design and carry out an experiment that is capable of testing the hypothesis. In other words, the experiment must be designed so that it will produce results that either clearly support or clearly falsify (disprove) the hypothesis. It helps to use “If-Then” predictions based on your hypothesis.

“Place 100 fruit flies at 18 degrees Celsius for one generation. Also place 100 fruit flies at 29 degrees Celsius for one generation. If the hypothesis is correct, then the fruit flies that develop at 18 degrees Celsius will complete their life cycle after those fruit flies that are placed at 29 degrees Celsius."

Step 4: Analyze Results and State Conclusions

Reject the hypothesis if the results are not consistent with the hypothesis or accept the hypothesis as possibly true if the results are consistent with the hypothesis. Notice that the hypothesis is not “proven to be true” even if the results do support it. This is because there may be explanations other than the hypothesis for the experimental result. 

For example, if the fruit flies placed at 18 degrees Celsius do develop slower, it may be that their food is not as soft making it more difficult for the fruit flies to eat at the lower temperature, causing them to eat less food and thus grow slower.

If the experimental results do not support the hypothesis, the hypothesis may be modified and additional experiments may be done to test the new or revised hypothesis. 

Figure \(\PageIndex{1}\). (CC BY-NC-SA)

Designing a Good Experiment

The most challenging part of the scientific method is usually the third step, designing and carrying out an experiment to test the hypothesis. A well-designed experiment should include all of the following characteristics:

1. An independent variable. The independent variable is the part of the experiment that the scientist changes or manipulates to see what effect occurs.

“The temperature is the independent variable, since that is what the experiment changes to see its effect.”

2. A dependent variable. The dependent variable is the part of the experiment that changes because of the change in the independent variable. In other words, the dependent variable is the effect that occurs from changing the independent variable.

“The length of the fruit flies' life cycle is the dependent variable, since the time of development is expected to change because of the temperature.”

3. An experimental group. The experimental group is the group of subjects where the independent variable is set to an unusual or test level.

“The fruit flies placed at 18 degrees Celsius are the experimental group, since the effect of lower temperature on the life cycle of fruit flies is what is being tested.”

4. A control group. A control group is the group of subjects in the experiment that the experimental group is compared to. For the control group, the independent variable is set to a normal or usual level (which may be zero, if that is considered a normal level).

“The fruit flies kept at 29 degrees are the control group, since this is the optimal temperature for fruit fly development.”

Some experiments include things called positive controls and negative controls. These are slightly different than control groups. Positive and negative controls serve to show that the experiment is working correctly. A positive control is a part of the experiment that is deliberately designed to give a positive result. It shows that the experiment is capable of producing a positive result when it is supposed to. A negative control is a part of the experiment that is designed to give a negative result. It shows that the experiment is capable of producing a negative result when it is supposed to.

5. The experiment should contain repetition. This means that there should be more than one subject in the experimental group and the control group. Why? In general, the more repetition, the less likely that your results are due to random chance.

“The experimental group and the control group each contained 100 fruit flies.”

6. The experiment should be well defined. One aspect of “well defined” is that the procedure (the steps) must be written down and clearly described. The true test of a well written experimental procedure is that another scientist could duplicate it exactly using just the written directions. Another aspect of “well defined” is that everything in the experiment, such as the materials, chemicals, equipment, environmental conditions, and the subjects (the organisms involved in the experiment), should be described as exactly as possible. All of the factors that are kept equal in the experiment and control groups are called standardized variables. Another aspect of “well defined” is that all parts of the experiment should be quantified. This means they should be measured by numbers. 

a) All fruit flies in this experiment are the same species (Drosophila melanogaster). All fruit flies were placed in plastic vials with standard cornmeal media and a cotton stopper. All fruit flies received artificial sunlight for 16 hours per day. 

b) One group of 100 fruit flies (the experimental group) were placed in an incubator set to 18 degrees Celsius.

c) One group of 100 fruit flies (the control group) were placed in an incubator set to 29 degrees Celsius.

d) In order to accurately determine the length of the life cycle of the fruit flies, 100 adult flies were allowed to lay eggs for 2 days and then removed from vials. Vials were monitored daily to determine the stage of the life cycle of the offspring of the original adult fruit flies. The end of the life cycle was recorded as the time when half of the fruit flies had undergone eclosion (adults emerge from the pupa case).

What is a Scientific Theory?

"It's just a theory." Our everyday use of the term theory often implies a mere guess or something that is unproven. However, a scientific theory implies that something has been proven and is generally accepted as being true. A scientific theory is an explanation for natural events that is based on a large number of observations and has been verified multiple times. Theories help scientists to explain large amounts of data. 

A theory differs from a hypothesis in that it is much broader in scope and supported by a much greater body of evidence. Remember, a hypothesis is an educated guess that is based upon observation, but has not yet been proven.

While theories are not easily discarded, as they are based on a large body evidence, sometimes scientists must modify or even reject scientific theories when new research methods produce results that do not fit with the current theory. 

 

Scientific Method Tutorial by Dr. Katherine Harris is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 3.0 Unported License.

This tutorial was funded by the Title V-STEM Grant #P031S090007.