DNA – September 20, 20101. Estimate the

greatest number of Yellow Fever cases in one year represented on this graph.

2. As the official coverage of immunization increased, what happened to the number of Yellow Fever cases?

3. What does the y-axis represent on this graph?

Variables

Ms. KlinkhachornSeptember 20, 2010

Biology

Variables

• A variable is something that can be changed in an experiment

• There are 3 kinds of variables that we will be looking for in our experiments:– Independent variable– Dependent variable– Controlled variables (also called constants)

Independent Variables

• This is the variable that the scientist purposely changes in the experiment.– We want to see how it affects the outcome– There should only be ONE per experiment. Why?

• Format: Broad Category ( ______ vs ______ )– Example: What is the effect of colored light on

plant height?– Colored Light (blue light vs green light vs red light)

Dependent Variables

• This is the variable that the scientist will measure– We want to see how it is affected by the change in the

independent variable

• Format: Broad Category (tool that we use to measure it)– Example: What is the affect of colored light on plant

height?– Height (meter stick)

Controlled Variables

• We also call them constants• These stay the same. • What would happen if they didn’t stay the same?– Example: If we are investigating how colored light

affects plant height, can we also water one of the plants more often or give one of the plants fertilizer?

– In this example, constants would be the type of plant, how often they are watered, the type of soil they are in, how long we let them grow, etc

Example 1

• Two groups of students were tested to compare their speed working math problems. Each group was given the same problems. One group used calculators, the other group computed without calculators.

Example 2• Scientist observed that white mice that were fed

seeds appeared to grow more than mice given vegetables. The scientists hypothesized that the protein in the seeds was responsible for the growth. They designed an experiment to test this hypothesis. They divided 200 mice of the same age, size, health, and sex into two groups of 100 mice each. The mice were kept under identical conditions for 14 days. One group was given a diet low in protein. The other group was given a normal protein diet. The mass of each mouse was recorded daily for 14 days.

Example 3

• Hillary wondered if the type of paper used to construct a paper airplane would affect the distance that the plane flew. She hypothesized that an airplane made of lighter weight paper would fly farther than an airplane made of heavier paper. Using three pieces of paper, each of a different weight, Hillary made three airplanes using the same design. She threw each plan five times, and measured the distance of each flight.