AP Biology Experimental Design

Experimental design and

Scientific Variables

AP-DC Biology

Science Practice (SP) 3.C

Experimental Design

★ SP 3.A: “Identify experimental procedures that are aligned to the question including: identifying the independent and dependent variables, identifying appropriate controls, and justifying appropriate controls ”

EXPERIMENTAL DESIGN and AP

On the AP test, you will be expected to:

○ Design experiments
○ Identify scientific variables (IV, DV, Controls, Constants, etc.
○ Explain the purpose of different aspects of a given experiment
○ Graph a set of given data
○ Make conclusions based on given experimental data

What is Experimental Design?

Experimental Design is the
process scientists use to plan
effective experiments that
utilize the scientific method

The Scientific Method

★ All good science starts with an

observation and a testable
question.

★ Prominent Examples

○

Why did that apple fall on my
head?

○

Why are no bacteria growing
around the mold in these agar
plates?

○

What would happen to our
perception of time if we were
traveling close to the speed of
light?

Experimental
Design 101

Experimental Design Must-Haves

All experiments require identification and inclusion of the following:

○ Hypothesis
○ Independent, Dependent and Constant Variables
○ Control Groups

What is a hypothesis?

●A prediction or possible answer to the problem or question you are

attempting to answer.

●Hypotheses always predict a relationship between the

independent and dependent variable (IV and DV).

●The best way to formulate a hypothesis is to ask yourselves these

questions:
○ 1. What is the independent variable from your observation/question
○ 2. What is the dependent variable from your observation/question
○ 3. What relationship would you expect to see based on your observation?

➔

“Normal” hypothesis

➔

An

educated

prediction

about the results of the
experiment.

➔

A statistical hypothesis

➔

States that the IV has no effect
on the DV

➔

The NH is assumed to be
correctunless

statistical

analysisof

the

data

contradicts it.

How do transcription factors know

where the gene is?

Null

Hypotheses (NH)

Alternative
Hypotheses (AH)

Types of
Hypotheses

Null hypotheses are not formatted
as “If…then…” statements.

“Studying for my classes will have
no affect on my grades”

For AP Biology, it is best practice
to write your hypothesis in an
“If...then….”

“If I study for my classes, then I
will achieve higher grades”

Hypothesis Practice

The rate of plant growth is directly related to the intensity of light received by the

chlorophyll molecules in the thylakoid membranes of the chloroplast. Scientists expose

different plants to different light intensities and measure plant growth.

Propose an alternative AND null hypothesis for this experiment.

AH: If the plant receives a higher intensity of light, then the plant will experience
higher growth rates.

NH: The intensity of light received by the plant will not affect its growth rate.

➔

The variable that is
manipulated or changed by
the researcher.

➔

This variable will vary
between experimental groups.

➔

The variable that changes in
response to the IV

➔

The DV is what is being
measured throughout an
experiment.

How do transcription factors know

where the gene is?

Dependent
Variable (DV)

Independent
Variable (IV)

Scientific Variables

Identify the Dependent Variable

A scientist is trying to determine if a
certain medicine kills cancer cells. They
treat the cancer cells with the medicine
and then measure how many die.

Identify the Independent Variable

A scientist is doing an experiment to
determine how light intensity affects
plant growth. They set up five plants in
different

intensities

of light

and

measure the growth of each plant.

Constant Variables

➔

Variables that remain the same in
every group of an experiment.

➔

Essential to ensure that the changes
in the DV are being caused by
manipulation of the IV and not some
other variable

➔

What are some constant variables you
would include in the experiment
shown to the right?

Types of Groups

found in
experiments

Experimental Design Groups

➔ Positive Control Group

◆

Group that receives the “normal” amount/type of the independent variable.

◆

Used to ensure that the test subjects are working properly, and to make
comparisons.

➔ Negative Control Group

◆

Group that does not receive the independent variable.

◆

Used to ensure that the test subjects are working properly, and to make
comparisons.

➔ Test/Experimental Groups

◆

Groups that receive various amounts/types of independent variable
treatments, that is different than the control group(s)

◆

Used to actually test the hypothesis

All experiments will have a control of

some kind, but NOT all experiments will
have a positive AND negative control.

Experimental Design Groups

The rate of plant growth is directly related to the intensity of light received by the

chlorophyll molecules in the thylakoid membranes of the chloroplast. Plants normally

receive 8-10 hours of sunlight a day, depending on the season. Scientists expose different

plants to different light intensities and measure plant growth.

Propose a positive control group, a negative control group, and TWO possible

experimental groups

Positive Control: A plant that receives 8-10 hours of sunlight.

Negative Control: A plant that receives no sunlight at all

Experimental Groups: Plants receiving 2 and 4 hours of sunlight

Any Questions?