McDougal Littell Science Cells and Heredity 

Table of Contents
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Determining Variables and Constants

EXPERIMENTAL GROUP AND CONTROL GROUP

An experiment to determine how two factors are related always has two groups—a control group and an experimental group.

  1. 1. Design an experimental group. Include as many trials as possible in the experimental group in order to obtain reliable results.
  2. 2. Design a control group that is the same as the experimental group in every way possible, except for the factor you wish to test.

Experimental Group: two containers of lake water with one drop of fertilizer solution added to each

Control Group: two containers of lake water with no fertilizer solution added

VARIABLES AND CONSTANTS

Identify the variables and constants in your experiment. In a controlled experiment, a variable is any factor that can change. Constants are all of the factors that are the same in both the experimental group and the control group.

  1. 1. Read your hypothesis. The independent variable is the factor that you wish to test and that is manipulated or changed so that it can be tested. The independent variable is expressed in your hypothesis after the word if. Identify the independent variable in your laboratory report.
  2. 2. The dependent variable is the factor that you measure to gather results. It is expressed in your hypothesis after the word then. Identify the dependent variable in your laboratory report.

Table 1. Variables and Constants in Algae Experiment

Independent
Variable
Dependent
Variable
Constants
Amount of fertilizer in lake water Amount of algae that grow
  • Where the lake water is obtained
  • Type of container used
  • Light and temperature conditions where water will be stored
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MEASURING THE DEPENDENT VARIABLE

Before starting your experiment, you need to define how you will measure the dependent variable. An operational definition is a description of the one particular way in which you will measure the dependent variable.

Your operational definition is important for several reasons. First, in any experiment there are several ways in which a dependent variable can be measured. Second, the procedure of the experiment depends on how you decide to measure the dependent variable. Third, your operational definition makes it possible for other people to evaluate and build on your experiment.

EXAMPLE 1

An operational definition of a dependent variable can be qualitative. That is, your measurement of the dependent variable can simply be an observation of whether a change occurs as a result of a change in the independent variable. This type of operational definition can be thought of as a “yes or no” measurement.

Table 2. Qualitative Operational Definition of Algae Growth

Independent Variable Dependent Variable Operational Definition
Amount of fertilizer in lake water Amount of algae that grow Algae grow in lake water

A qualitative measurement of a dependent variable is often easy to make and record. However, this type of information does not provide a great deal of detail in your experimental results.

EXAMPLE 2

An operational definition of a dependent variable can be quantitative. That is, your measurement of the dependent variable can be a number that shows how much change occurs as a result of a change in the independent variable.

Table 3. Quantitative Operational Definition of Algae Growth

Independent Variable Dependent Variable Operational Definition
Amount of fertilizer in lake water Amount of algae that grow Diameter of largest algal growth (in mm)

A quantitative measurement of a dependent variable can be more difficult to make and analyze than a qualitative measurement. However, this type of data provides much more information about your experiment and is often more useful.