Introduction to Experimental Design

AP Biology

ScienceA way of learning

about the natural worldIncludes all of the

knowledge gained by exploring the natural world

This knowledge is always growing and changing as scientists ask new questions and explore new ideas

Scientific InquiryScientific: Of or relating to the practice of

scienceInquiry: A close examination of a matter in a

search for information or truthScientific Inquiry: The ways in which

scientists explore the natural world

How Does Scientific InquiryWork?Scientific Method:

Make observations and/or inferences

Ask questionsDevelop hypothesesDesign experimentsMake measurementsCollect dataInterpret dataDraw conclusionsCommunicate

Make Observations and InferencesObservation: Using one or more of your

senses to collect dataInference: An interpretation of an

observation that is based on evidence or prior knowledge

Data: Facts, figures and other evidence gathered through observation

QuestionYou ask a question

about what you observe.

State the problem or question. Example: Does the

drop height affect the bounce height of a superball?

Developing a HypothesisHypothesis: A possible explanation for a set

of observations or answer to a scientific question

A hypothesis must be something that can be tested

Worded as an If . . . Then statementHypotheses can either be supported or

disprovedExample: If a superball is dropped from

increasing heights then the bounce heights will also increase because…

Null HypothesisA hypothesis that can be rejected-it

essentially states that the variables will have no effect on the outcome of the experiment

Why use a Null Hypothesis?You can never have enough support for a

hypothesis to say that it is the sole explanation of a phenomenon, but you can have enough data to show that it is not the sole explanation of a phenomenon.

Designing an experiment totest a hypothesisAfter you state your hypothesis, you must

design an experiment to test it.You figure out a way to test whether the

hypothesis is supported. Control group– nothing changed Experimental group – one variable

changedThe outcome must be measurable

(quantifiable).Record and analyze data.

Designing a Controlled Experiment.An experiment in which all of the variables

except for one are remain the sameAn experiment that is designed to compare an

experimental group with a control group.Ideally, a control group and an experimental

group differ in only one variable.Control group serves as a comparison to

measure against

Experiment VariablesVariable: Factors that can change in an

experimentManipulated (Independent) Variable: The

variable that is changed during the experiment

Responding (Dependent) Variable: The factor that changes because of the manipulated variableVariable that is measured in the experiment

VariablesIndependent variable

Example: Does the drop height affect the bounce height of a superball? We know the drop heights we will use.

Dependent variableExample: Does the drop height affect the

bounce height of a superball? We don’t know the bounce height before we start.

It is what is being measured.

ConstantsConstant: a variable that does not change

for the duration of an experiment; a value that remains the sameExamples: Does the drop height affect the

bounce height of a superball? The superball does not change during the

experiment. The same force is applied during each trial.

Interpreting DataYou do the experiment using

the method you came up with and record the results.

After the data are collected, they must be organized and interpreted Create data tables,

graphs to represent dataPerform calculations that

will determine what results mean (averages, percentages, totals)

Identify possible sources of errors

Drawing ConclusionsAfter you have

organized and interpreted your data, you must draw a conclusion.

You need to decide if the data support your original hypothesis.

RetestIn order to verify the results, experiments

must be retested.

CommunicatingScientists must communicate their findings to

other scientists. This contributes to the growth of the body ofscientific knowledge.

Scientific Theories

Scientific Theory: A well-tested concept that explains a wide range of observations.

A theory is based on thousands of experiments done by different scientists. A theory is much broader in scope than a

hypothesis. Theories only become widely accepted in science if

they are supported by an accumulation of extensive and varied evidence.

Future testing may prove a theory to be incorrect.