Reference Sheet - CLASS SET!
Experimental Research: Basic Principles
Science relies heavily on the use of experiments. The purpose of experimental research (doing experiments) is
to find out what causes things to happen (to determine cause-effect relationships). Experimental research allows
scientists to explain the things they study (that is, to be able to say why something happens).
A. Experiments involve independent variables.
 In experiments, one variable is changed to see what effect that change causes.
o A researcher interested in the effect of soil temperature on plant growth would change the soil
temperature and then determine the effect of that change on plant growth.
o A researcher interested in the effect of vitamin C on resistance to colds would change the dosage of
vitamin C given to people and then determine the effect of that change on cold resistance.
 The variable that is changed is called an independent variable (I.V.) or manipulated variable.
o In an experiment studying sunlight’s effect on plant growth, sunlight is the independent variable.
o In an experiment studying the effect of noise on student test performance, noise is the I.V.
o In an experiment studying the effect of radiation on tumor growth in mice, radiation is the I.V.
 In order to study the effect of an independent variable, the experimenter creates multiple
experimental conditions where the variable must be changed or manipulated in some way so that
different levels of the variable are created. Only by comparing the effects of these different levels can
the experimenter determine the influence of an independent variable. Here are some examples of how an
experimenter might manipulate an independent variable:
o In an experiment testing the effect of sunlight on plant growth, the experimenter could vary the
amount of time that a plant received sunlight each day. For example, some plants might receive 2 hours
of sunlight, some 4 hours, and others 8 hours.
o In an experiment testing the effect of noise on test performance, the experimenter could vary noise
level (intensity). For example, the two levels might be: (1) high intensity (95 decibels), and (2) low
intensity (40 decibels).
o In an experiment studying the influence of radiation on tumor growth in mice, the researcher could
vary the level of radiation (a high versus a low dosage).
B. Experiments involve dependent variables.
 A dependent variable (D.V.) is the variable that is measured – it is the effect or result of the
independent variable. Here are some examples:
o
In an experiment studying sunlight’s effect on plant growth, the dependent variable is plant growth.
o
In an experiment studying the effect of noise on worker performance, the D.V. is worker
performance.
o
In an experiment studying the effect of radiation on tumor growth in mice, tumor growth is the D.V.
o
In an experiment studying how nicotine level in tobacco affects addiction to smoking, addiction = D.V.
 In any experiment, the investigator must determine how to measure the dependent variable. Using the
previous examples, here are some strategies an experimenter can use:
o In an experiment studying the effect of sunlight on plant growth, an experimenter could measure plant
growth by taking daily measurements of the height and width of the plants. The experimenter should be
consistent in their measurements (measure at the same time each day – plant width may vary, measure the
same part of the same plant, i.e. – just below the first leaf).
o In an experiment studying how noise affects worker performance, performance could be measured in
several ways: (1) number of products made in a period of time, (2) errors made during a work period, or (3)
time taken to complete a work task.
o In an experiment on the effect of radiation on tumor growth of mice, an experimenter might measure
tumor growth by determining the size of the tumor from an x-ray.
o In an experiment studying the effect of nicotine level on addiction, addiction could be measured by
counting the number of cigarettes a person smoked during a 24-hour period.
C. Experiments are designed to test a hypothesis.
 A hypothesis is an educated guess about how the independent variable will affect the dependent
variable. A hypothesis is often an if/then statement that should be testable so that the experimenter can
determine the effect of the independent variable on the dependent variable. A hypothesis should also be
clear about what measurements the experimenter will need to make. Here are some examples:
o If sunlight increases plant growth, then plants that receive longer periods of sunlight will grow taller.
o If noise reduces worker performance, then workers subjected to increased levels of noise (decibels) will
produce fewer products in one hour.
D. Experiments involve controlled variables/constants.
 In experiments, only one independent variable can be changed. All other potential variables need to
be controlled (kept the same or constant). Therefore the influence of such extra variables is eliminated.
This means that any difference in the dependent variable that occurs during the experiment must be due to
the independent variable. This is what allows an experimenter to say the independent variable caused the
effect on the dependent variable. Variables that are kept constant are called controlled
variables/constants.
o Suppose an experimenter is investigating the effect of study method on test performance. The
experimenter must hold constant such variables as amount of study time or time allowed for test taking
that might have some influence on test performance.
o In an experiment testing the effect of sunlight on plant growth, the researcher must control factors
other than sunlight that might influence plant growth – factors such as room temperature, humidity level,
and amount of water given to the plants.
o Consider this example in which outside factors are not controlled: A scientist wants to know the effect
of a low-fat diet on a monkey’s physical health (for example, the ability to resist infection). The scientist
studies 100 monkeys. Half the monkeys are fed a low-fat diet, and half are fed a high-fat diet for six
months. However, the monkeys are actually fed different foods, so not only do the diets differ in fat,
they are different in other characteristics such as iron, protein, and calcium. If the two different groups
of monkeys end up different in terms of physical health, the scientist will not know if the difference is
due to the fat content of the diet or to other diet differences. The scientist could solve the problem by
giving all 100 monkeys the same basic food; for half of the monkeys, however, the fat content is lowered.
E. Experimental Procedures

An experimental procedure is a series of at least three steps that are clear and easy to follow.
Well written procedures include specific information for manipulating the independent variable and
collecting data on the change in the dependent variable. An example follows:
Hypothesis: If fertilizer effects plant growth, then plants grown with fertilizer will grow taller than those
grown without fertilizer.
Procedure:
o Obtain 6 tomato plants that are each 6 inches tall. Keep each of them in identical clay pots placed
under a sun light for 16 hours each day.
o Water each plant with 100 mL of water every other day. For three of the plants add 1 teaspoon of
fertilizer to the water, but do not add fertilizer to the water for the other three plants.
o Each week measure the height of each plant and record your data in a table. Continue this for 4
weeks.