The Scientific Method
The Scientific Method
The following steps make up the Scientific Method. These steps make up a method which may be used to logically solve problems in many other areas of life. Françesco Redi and Louis Pasteur used the scientific method to disprove the idea of spontaneous generation.
First, though, do you see any animals in this picture? (if so, click on them)
If you were really in that place and trying to figure out what you were seeing, you could use the scientific method to study the “problem.”
(There is a link to an explanation of the photograph near the bottom of this page.)
Observation:A good scientist is observant and notices thing in the world around him/herself. (S)he sees, hears, or in some other way notices what’s going on in the world and becomes curious about what’s happening.
This can and does include reading and studying what others have done in the past because
scientific knowledge is cumulative. In physics, when
Newton came up with his Theory of Motion, he based his hypothesis on the work of
Copernicus, Kepler, and Galileo as well as his own, newer observations. Darwin not only observed and took notes
during his voyage, but he also studied the practice of artificial selection and read the works of other naturalists to form his Theory of Evolution.
For centuries, people based their beliefs on their interpretations
of what they saw going on in the world around them without testing their
ideas to determine the validity of these theories — in other words, they
didn’t use the scientific method to arrive at answers to their
questions. Rather, their conclusions were based on untested observations.
Among these ideas, since at least the time of Aristotle
(4th Century BC),
people (including scientists) believed that simple living organisms could
come into being by spontaneous generation. This was the idea that
non-living objects can give rise to living organisms. It was
common “knowledge” that simple organisms like worms, beetles,
frogs, amd salamanders could come from dust, mud, etc., and food left out,
quickly “swarmed” with life. For example:
Observation: Every year in the spring, the Nile River flooded areas
of Egypt along the river, leaving behind nutrient-rich mud that enabled the
people to grow that year’s crop of food. However, along with the muddy soil,
large numbers of frogs appeared that weren’t around in drier times.
“Conclusion”: It was perfectly obvious to people back then that
muddy soil gave rise to the frogs.
Observation: In many parts of Europe, medieval farmers stored
grain in barns with thatched roofs (like Shakespeare’s house). As a roof aged,
it was not uncommon for it to start leaking. This could lead to spoiled or
moldy grain, and of course there were lots of mice around.
“Conclusion”: It was obvious to them that the mice came from the
moldy grain.
Observation: In the cities, there were no sewers nor garbage
trucks. Sewage flowed in the gutters along the streets, and the sidewalks
were raised above the streets to give people a place to walk. In the
intersections, raised stepping stones were strategically placed to allow
pedestrians to cross the intersection, yet were spaced such that carriage
wheels could pass between them. In the morning, the contents of the chamber
pots were tossed out the nearest window. When people were done eating a meal,
the bones were tossed out the window, too. A chivalrous gentleman always
walked closest to the street when escorting a woman, so if a horse and
carriage came by and splashed up this filth, it would land on him, and not
the lady’s expensive silk gown. Most of these cities also had major rat
problems which contributed to the spread of Bubonic Plague (Black
Death) — hence the story of the Pied Piper of Hamelin, Germany.
“Conclusion”: Obviously, all the sewage and garbage turned into
the rats.
Observation: Since there were no refrigerators, the mandatory,
daily trip to the butcher shop, especially in summer, meant battling the
flies around the carcasses. Typically, carcasses were “hung by their
heels,” and customers selected which chunk the butcher would carve off
for them.
“Conclusion”: Obviously, the rotting meat that had been hanging in
the sun all day was the source of the flies.
From this came a number of interesting recipes, such as:
Recipe for bees:Kill a young bull, and bury it in an upright
position so that its horns protrude from the ground. After a month, a swarm
of bees will fly out of the corpse.
Jan Baptista van Helmont’s recipe for mice:Place a dirty shirt
or some rags in an open pot or barrel containing a few grains of wheat or
some wheat bran, and in 21 days, mice will
appear. There will be males and females present, and they will be
capable of mating and reproducing more mice.
With the development and refinement of the microscope in
the 1600s, people began seeing all sorts of new life forms such as yeast and
other fungi, bacteria, and various protists. No one knew from where these
organisms came, but people figured out they were associated with things like
spoiled broth. This seemed to add new evidence to the idea of spontaneous
generation — it seemed perfectly logical that these minute organisms should
arise spontaneously. When Jean Baptiste Lamarck proposed his theory of
evolution, to reconcile his ideas with Aristotle’s Scala naturae, he
proposed that as creatures strive for greater perfection, thus move up the
“ladder,” new organisms arise by spontaneous generation to fill the
vacated places on the lower rungs.
Observations: It was
known that soup that was exposed to the air spoiled — bacteria grew in it.
Some people claimed that there was a “life force” present in the molecules
of all inorganic matter, including air and the oxygen in it, that could
cause spontaneous generation to occur, thus accounting for the presence of
bacteria in spoiled soups. Even when briefly-boiled soup was poured
into “clean” flasks with cork lids, microorganisms still grew there.
Containers of soup that had been boiled for one hour, and then were sealed,
remained sterile. Boiling for only a few minutes was not enough to sterilize
the soup.
Question:The scientist then raises a question about what (s)he sees going on.
The question raised must have a “simple,” concrete answer that can be obtained by performing
an experiment. For example, “How many students came to school today?” could be answered by counting the students present on campus,
but “Why did you come to school today?” couldn’t really be answered by doing an experiment.
Question: Where do the flies at the butcher shop really come from? Does rotting meat turn into or produce the flies?
Question: Is there indeed a “life force” present in air
(or oxygen) that can cause bacteria to develop by spontaneous generation? Is
there a means of allowing air to enter a container, thus any life force, if
such does exist, but not the bacteria that are present in that air?
Hypothesis:
This is a tentative answer to the question: a testable explanation for what was observed. The scientist tries to explain what caused what was observed.
Hypothesis: Rotten meat does not turn into flies. Only flies can make more flies.
Hypothesis: There is no such life force in air, and a container of
sterilized broth will remain sterile, even if exposed to the air, as long as
bacteria cannot enter the flask.
In a cause and effect relationship, what you observe is the effect, and hypotheses are possible causes. A generalization based on inductive reasoning is not a hypothesis.
An hypothesis is not an observation, rather, a tentative explanation for the observation.
For example, I might observe the effect that my throat is sore.
Then I might form hypotheses as to the cause of that sore throat, including a bacterial infection, a viral infection, or screaming too much at a ball game.
Hypotheses reflect past experience with similar questions (“educated propositions” about cause) and the work of others.
Hypotheses are based on previous knowledge, facts, and general principles.
Your answer to the question of what caused the observed effect will be based on your previous knowledge of
what causes similar effects in similar situations. For example, I know that colds are contagious, I don’t know
anyone with a cold, I was at the ball game yesterday, and I was doing a lot of yelling while I was
there, so I think that caused my sore throat.
Multiple hypotheses should be proposed whenever possible. One should think of alternative causes that could explain the observation (the correct one may not even be one that was thought of!)
For example, maybe somebody sitting near me at the ball game had a sore throat and passed it on to me.
Hypotheses should be testable by experimentation and deductive reasoning.
For example, throat culture and other
tests yielded no signs of a bacterial or viral infection, I have no fever or other signs/symptoms, and the doctor says
my vocal cords are “swollen” in a way that would indicate overuse.
Hypotheses can be proven wrong/incorrect, but can never be proven or confirmed with absolute certainty.
It is impossible to test all given conditions, and someone with more knowledge, sometime in the future, may find a condition under which the hypothesis does not hold true.
Prediction:Next, the experimenter uses deductive reasoning to test the hypothesis.
Prediction: If meat cannot turn into flies, rotting meat in a sealed (fly-proof) container should not produce flies or maggots.
Prediction: If there is no life force, broth in swan-neck flasks
should remain sterile, even if exposed to air, because any bacteria in the
air will settle on the walls of the initial portion of the neck. Broth in
flasks plugged with cotton should remain sterile because the cotton is able
to filter bacteria out of the air.
Inductive reasoning goes from a set of specific observations to general conclusions: I observed cells in x, y, and z organisms, therefore all animals have cells.
Deductive reasoning flows from general to specific. From general premises, a scientist would extrapolate to specific results: if all organisms have cells and humans are organisms, then humans should have cells. This is a
prediction about a specific case based on the general premises.
Generally, in the scientific method, if a particular hypothesis/premise is true and “X” experiment is done, then one should expect (prediction) a certain result. This involves the use of “if-then” logic.
For example, if my hypothesis that my throat is sore because I did too much screaming at the ball game is true and if a doctor examines my vocal cords, then (s)he should be able to observe that they are inflamed, and as the inflammation heals, the sore throat should go away.
A prediction is the expected results if the hypothesis and other underlying assumptions and principles are
true and an experiment is done to test that hypothesis. For example, in physics if Newton’s Theory of
Motion is true and certain “unexplained” measurements and calculations pointing to the possibility
of another planet are correct, then if I point my telescope to the specific position that I can calculate
mathematically, I should be able to discover/observe that new planet. Indeed, that is the way in which Neptune was discovered in 1846.
Testing:Then, the scientist performs the experiment to see if the predicted results are obtained. If the expected results are obtained, that supports (but does not prove) the hypothesis.
In science when testing, when doing the experiment, it must be a controlled experiment. The scientist must contrast an “experimental group” with a “control group”. The two groups are treated EXACTLY alike except for the ONE variable being tested.
Sometimes several experimental groups may be used. For example, in an experiment to test the effects of day length on plant flowering, one could compare normal, natural day length (the control group) to several variations (the experimental groups).
When doing an experiment, replication is important. Everything should be tried several times on several subjects. For example, in the experiment just mentioned, a student scientist would have at least three plants in the control group and each
of the experimental groups, while a “real” researcher would probably have several dozen. If a scientist had only one plant in each group, and one of the plants died, there probably would be no way of determining if the cause of death was related to the
experiment being conducted.
The experimenter gathers actual, quantitative data from the subjects. For example, it’s not enough to say, “I’m going to see how the dog reacts in this situation.” Rather, in that experiment, the scientist might have a list of certain
behaviors, and record how often each of the dogs tested exhibits each of those pre-defined behavior patterns. Data for each of the groups are then averaged and compared statistically. It’s not enough to say that the average for group “X” was one thing and the average
for group “Y” was another, so they were different or not. The scientist must also calculate the standard deviation or some other statistical analysis to document that any difference is statistically significant.
Testing: Wide-mouth jars each containing a piece of meat were
subjected to several variations of “openness” while all other
variables were kept the same.
control group — These jars of meat were set out without lids so the meat
would be exposed to whatever it might be in the butcher shop.
experimental group(s) — One group of jars were sealed with lids, and
another group of jars had gauze placed over them.
replication — Several jars were included in each group.
Data: Presence or absence of flies and maggots seen in each jar
was recorded. In the control group of jars, flies were seen entering the
jars. Later, maggots, then more flies were seen on the meat. In the
gauze-covered jars, no flies were seen in the jars, but were observed around
and on the gauze, and later a few maggots were seen on the meat. In the
sealed jars, no maggots or flies were ever seen on the meat.
Conclusion(s): Only flies can make more flies. In the uncovered
jars, flies entered and laid eggs on the meat. Maggots hatched from these
eggs and grew into more flies. flies laid eggs on the gauze on
the gauze-covered jars. These eggs or the maggots from them dropped through the
gauze onto the meat. In the sealed jars, no flies, maggots, nor eggs could
enter, thus none were seen in those jars. Maggots arose only where flies
were able to lay eggs. This experiment disproved the idea of spontaneous
generation for larger organisms.
Testing: Broth was boiled in various-shaped flasks to sterilize
it. As the broth and air in the containers cooled, fresh
room air was drawn into the containers. None of the flasks were sealed — all
were exposed to the outside air in one way or another.
control group — Some flasks opened straight up, so not only air,
but any bacteria present in that air, could get into them.
experimental group(s) — Some flasks had long, S-shaped
necks (swan-neck flasks) and others were “closed” with cotton plugs. This allowed
air to enter these flasks, but the long, swan neck or the cotton balls
filtered out any bacteria present in that air. The
long necks were subsequently broken off some of the swan-neck flasks.
replication — Several flasks were used in each of the groups.
Data: Broth in flasks with necks opening straight up
spoiled (as evidenced by a bad odor, cloudiness in previously clear broth,
and microscopic examination of the broth confirming the presence of bacteria),
while broth in swan-neck flasks did not, even though fresh air could
get it. Broth in flasks with cotton plugs did not spoil, even though air
could get through the cotton. If the neck of a swan-neck flask was broken
off short, allowing bacteria to enter, then the broth became contaminated.
Conclusion(s): There is no such life force in air, and organisms
do not arise by spontaneous generation in this manner. To quote Louis
Pasteur, “Life is a germ, and a germ is Life. Never will the doctrine of
spontaneous generation recover from the mortal blow of this simple experiment.”
Research is cumulative and progressive. Scientists build on the work of previous researchers, and one important part of any good research is to first do a literature review to find out what previous research has already been done in the field.
Science is a process — new things are being discovered and old, long-held theories are modified or replaced with
better ones as more data/knowledge is accumulated. For example, the idea that the sun is at the center of our solar system replaced the idea that the earth was at the center of the universe, and the idea that
ulcers are caused by stress has been replaced by the idea that ulcers are caused by bacterial infection. Scientists are human, too, and so these major changes are often controversial and accompanied by violent debate!
A theory is a generalization based on many observations and experiments; a well-tested, verified
hypothesis that fits existing data and explains how processes or events are thought to occur. It is
a basis for predicting future events or discoveries. Theories may be modified as new information is gained.
This definition of a theory is in sharp contrast to colloquial usage, where people say something is “just a theory,” thereby intending to imply a great deal of uncertainty.
Observation: Have you ever noticed if you place a plant near a window, that after a while, the plant grows or leans toward the window?
Question: Have you ever wondered why the plant grows toward the window?
Hypothesis: What reasons or factors can you list that might cause a plant to lean or grow toward a window? For example, do you think that
plants like Microsoft products
plants catch a virus from the window, and that makes them lean toward the window
plants respond to the glass from which they absorb some needed nutrient
plants respond to the light which they need to make their food
plants respond to fresh air leaking in the window
plants are attracted to the plants outside the window
Prediction: If it is true that
and I
grow a plant in front of my computer
grow a plant in front of a PC and one in front of a Mac
grow three plants in front of a PC and three in front of a Mac
grow a plant under a light
grow three plants under a light
grow a plant under a regular light and one under a red light
grow one plant with no added fertilizer and another with 1 T of fertilizer added
grow a plant in the dark
grow a plant in a glass cube in the dark
grow three plants under a light and three plants in the dark
grow three plants with a light to the left of them and three plants with a light to the right of them
grow three plants with a light to the left of them and a piece of glass to the right
grow three plants with light to the left and glass to the right and three more with light to the right and glass to the left
grow three plants with glass on the left and a fan on the right
grow three plants with a light to the left and glass to the right and three plants in the dark with glass on the right
grow three plants with a light to the left and a fan on the right
grow three plants with light to the left and glass to the right and three more with a fan to the left and other plants to the right
grow three plants with other plants on the left and glass on the right and three others with plants on the right and glass on the left
grow three plants outdoors in fresh air and three plants indoors
grow three plants with glass on the left and three with glass on the right
grow 100 plants in 20° C air and 100 plants in 37° C air, and 50 of each have light to the left while the other 50 of each have glass to the left
grow three plants in glass cubes in the light and three plants in glass cubes in the dark
grow three plants on motorized turntables with a light to one side and three plants that are stationary with a light to one side
grow three plants on motorized turntables with glass on one side and three plants that are stationary with glass on one side
grow ten plants in one greenhouse and ten more plants, each in an individual greenhouse
then I should see a difference or change in the
soil nutrients
leaf color
number of leaves
length of the stem
number of blossoms
direction the plant is growing/leaning
Review the steps in the Scientific Method
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(2.2 MB) Click the picture to start. Press [ESC] to stop the presentation or click on the presentation to re-start it. You may also “write” on the presentation. Unfortunately, Corel only has a Plug-In for Win 95/NT, so this won’t work with Win 3.1 or Mac.
Sometimes, it doesn’t go this way. Sometimes serendipity (Serendib = former name for Ceylon) happens. The Persian fairy-tale The Three Princes of Serendip illustrates the principle known as serendipity. In this story, three
princes make discoveries by insight into accidents pertaining to things they were not seeking. Serendipity is not discovery just by accident alone, but includes the idea that the investigator has intuition, or knowledge, which enables him/her to
recognize and take advantage of unexpected events unrelated to his/her original quest. The discovery of aspartame is a good example of serendipity, but also an example of very bad lab technique. A chemist at Searle Chemical Company had his coffee cup
sitting on the benchtop in the chemistry lab next to his experiment. Somehow in the process of doing his experiment and drinking coffee all at the same time (not a good idea if you value your life), he stuck his fingers in his experiment, then into his
mouth. The serendipity comes in when he realized that this sweet-tasting accident could make his company and him rich.
To give you an idea of how the scientific method works, your study group is asked to go through the steps we just discussed as though you were real biologists getting ready to do real research. You will be doing all of the background work and
designing the experiment, but not actually doing it since this is not a lab course. However, you are asked to do a write-up of the experiment as though you had done it. For more information on this, refer to the Assignment on
Scientific Method that was handed out along with your syllabus.
References:
Alcamo, I. Edward. 1997. Fundamentals of Microbiology, 5th Ed. Benjamin Cummings Publ. Co., Menlo Park, CA.
Borror, Donald J. 1960. Dictionary of Root Words and Combining Forms. Mayfield Publ. Co.
Campbell, Neil A., Lawrence G. Mitchell, Jane B. Reece. 1999. Biology, 5th Ed. Benjamin/Cummings Publ. Co., Inc. Menlo Park, CA. (plus earlier editions)
Campbell, Neil A., Lawrence G. Mitchell, Jane B. Reece. 1999. Biology: Concepts and Connections, 3rd Ed. Benjamin/Cummings Publ. Co., Inc. Menlo Park, CA. (plus earlier editions)
Marchuk, William N. 1992. A Life Science Lexicon. Wm. C. Brown Publishers, Dubuque, IA.
Tell me about the picture of the bananas!
carterjs@uc.edu
Copyright © 1996 by J. Stein Carter. All rights reserved.
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Selected by the SciLinks program, a service of National Science Teachers Association. Copyright © 2001.
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