Variables Designing a fair experiment

What is a Variable? Scientists design experiments in which changes to one item cause something else to change in a predictable way. A variable is any factor, trait, or condition that can exist in differing amounts or types. An experiment usually has three kinds of variables: Controlled Independent Dependent

Independent Variables The independent variable is purposefully changed by the scientist A good, fair test only has one independent variable Why is it important to only have one independent variable?

Dependent Variables The dependent variable is caused by and depends on the independent variable. The scientist focuses his/her observations on how the dependent variable changes in response to the independent variable. Why is it necessary to observe the dependent variables?

Example  The longer the amount of time spent studying causes an increase in test scores  Remember – independent variable is changed purposefully and the dependent variable changes in response  Independent Variable – time spent studying  Dependent Variable – test scores

Controlled Variables  Controlled variables are values that a scientist wants to remain constant  They must be observed as closely as the dependent variables in order to maintain a fair test  There is usually more that one controlled variable  Controlled variables are also known as constant variables  Why does an experiment need controlled variables?

Variables  In a good experiment, the independent and dependent variables are both measurable.  Time, mass, length are all examples of measurable values  Variables with no true unit of measure would be a poor variable to use in an experiment (i.e. – love, success)‏

More Examples… Stirring Type of sugar Amount of sugar that dissolves completely measured in grams Temperature of water measured in degrees Celcius Does heating a cup of water allow it to absorb more sugar? The faucet Water pressure Amount of water flowing per minute measured in mL per minute Water faucet opening closed half open fully open How much water flows through a faucet at different openings? Controlled Variables (What is kept the same)‏ Dependent Variable (What is observed)‏ Independent Variable (What is change)‏ Question

Your turn…. After studying about recycling, members of John’s Biology class investigated the effect of various recycled products on plant growth. John’s lab group compared the effect of different aged grass compost on bean plants. Each plant received the same amount of sunlight and water each day. Identify the controlled, independent and dependent variables.

Answers… Independent Variable Amount of compost Dependent Variable Height of bean plant Controlled Variables Amount of sunlight Amount of water

The Role of Variables in the Scientific Method  Variables play an important part in the scientific method.  A hypothesis is formatted according to the independent and dependent variables.  A graph (data analysis) is labeled according to the independent and dependent variables  Variables are further explained in the conclusion.

Hypothesis  A hypothesis is a possible answer to a scientific question. It is NOT an educated guess.  A hypothesis should always be presented in the form of an “If…….then….. statement.”  “If” is followed by the independent variable, “then” is followed by the dependent variable.  Example - If a plant is grown in compost, then it will grow taller.

Hypothesis Practice  Independent Variable – time spent studyingDependent Variable – test grade  Independent Variable – light or darkDependent Variable - # of earthworms  Independent Variable – fertilizerDependent Variable – height of plant