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.