Identifying Variables

Variables  Recall: In an experiment, a variable is anything that can change. Scientists usually manipulate one variable, and measure a response in another.  Examples:  Temperature  Growth  Rates of change (rate of speed, rate of melting)  Time

Measured Variables  The manipulated variable is called the independent variable.  Example: If you were measuring how watering affects the growth of a plant…  How much water you add is the independent variable.

Measured Variables  The responding variable is called the dependent variable.  Example: If you were measuring how watering affects the growth of a plant…  The growth of the plant is the dependent variable.

Tips to Remember Variables  The independent is the cause. It is not affected by any other variable.  The dependent variable is the effect. It depends on the other variable.

Control Variables  Control variables are other things that could affect the experiment.  They must be kept constant to make sure they don’t have an effect.  What would we need to control in the watering plants experiment?  Light, location, type of plant, type of soil

Example  You are trying to determine which cola is preferred by students, Pepsi or Coke.  What are your independent and dependent variables?  Independent: Type of cola  Dependent: How much people enjoy it  What control variables do you need to consider?  Cola temperature, amount, presentation (i.e. cup), order of sampling, cleansing of palate, etc

Important Note  A control variable and a control group are completely different things. The definition of a control group is given on the next page of your notes – don’t get them confused!

Analyzing a Controlled Experiment  Definition 1: In experiments where we are changing the independent variable to investigate if there is a change from “the norm”, it is important to have data about this “normal group” to use as a baseline for comparison.  We typically call this baseline or normal group a control group.  In situations where only trial/data set is collected it is sometimes also called a control test.

Analyzing a Controlled Experiment  Definition 2: A placebo is a substance containing NO medication that is sometimes given to patients to reinforce their expectation of getting well. As far as the patient is concerned, however, he/she is getting the actual medication.

Applying What You’ve Learned  The disease “homeworkitis” affects 99% of Grade 9 students at St. Joan of Arc CSS. Students infected with the disease typically take 10 days to recover. A medical researcher wants to test the effectiveness of a new drug that is designed to cut an infected student’s recovery time in half (to 5 days).

She performs the following steps:  1) She gives a group of 10 infected students (test group 1) the experimental drug.  2) She gives another group of 10 infected students (test group 2) a placebo, while telling them that they are actually receiving the drug.  3) She observed the students over a period of five days.  4) After five days, she finds that students in both groups have completely recovered from their bout of homeworkitis!

Questions  What was the independent variable?  Whether the drug or the placebo was given  What was the dependent variable?  How quickly each student’s condition improves

Questions  Which test group was the control group?  Those who received the placebo  Was the experimental drug effective? Why?  No, since the control group also improved it suggests the improvement was due to a psychological effect and not the drug.