Designing a Controlled Experiment

Use this PPT to review the design of a controlled experiment.

PurposeThe purpose of a controlled experiment is to establish a cause and effect relationship

between two variables.

ObservationEvery experiment begins with an

observation.

Most dairy cows are milked two to three times per day. On average, a

cow will produce six to seven gallons of milk each day.

QuestionOne overambitious farmer wants his cows to produce more milk so that he can make more money. Since cow growth hormones are known to increase cow mass, he wonders if they can also increase

milk production.

QuestionThe question in a controlled

experiment is written in such a way as to include the cause

and effect.

How do growth hormones affect milk production in cows?

QuestionsScientific questions can take

many forms. For this class, I’d like you to consistently use the format on the previous slide.

HypothesisThe hypothesis in a controlled experiment again references

the cause and effect AND includes a specific prediction

about what will occur.If growth hormone IS USED, then milk

production in cows WILL INCREASE.

Controlled ExperimentSet up at least two groups.EXPERIMENTAL SET-UP / GROUP

CONTROLSET-UP / GROUP

15 cows getting the treatment

15 cows NOT getting the treatment

NOTE: The control set-up or the control group is commonly referred to as “the control.”

Controlled ExperimentContains 3 types of variables:

1. Independent Variable = condition that is being modified by the experimenter [CAUSE]

2. Dependent Variable = condition that changes as a result [EFFECT]

3. Controlled Variables = all the conditions that are kept constant between the experimental and control groups.

Controlled ExperimentCONTROL

SET-UP / GROUPEXPERIMENTAL SET-UP / GROUP

15 cows NOT getting the treatment

15 cows getting the treatment

IV = 15 cc of growth hormone daily 1 hour before milking

DV = amount of milk production

CONTROLLED VARIABLESage of cows, size of cows, diet, water, exercise

Data CollectionDecide what data to collect AND create

a data table BEFORE you begin!Day 1

Cow Number Experimental(Liters of milk produced daily)

Control(Liters of milk produced daily)

1

2

3

4

5

6

AVERAGE

Graphing ResultsCreate an appropriate graph of the results. In our class it will

usually be a line graph.

1 2 3 4 5 6 7 8 9 100

1

2

3

4

5

6

7

8

9

Influence of Growth Hormone on Milk Produc-tion in Cows

Experimental Group AverageControl Group Average

Days

Aver

age

Milk

Pro

ducti

on (L

)

Analysis > ConclusionAt this point the experimenter

tries to make sense of the data in order to reach a conclusion.

The conclusion answers the experimental question.

Growth hormone increases milk production in cows.

InferenceSometimes inference is used when

formulating conclusions.Inference = An inference is a reasonable conclusion derived by using existing knowledge to explain indirect observations.

“Because the street is wet, I infer that it rained”

* Add this term and its definition to your notes *

DiscussionIn a discussion, the experimenter describes the topic, experimental

procedure, conclusion, data support, further explanation, hypothesis

check, possible sources of error and suggestions for follow-up

experiments.

DiscussionDON’T WORRY ABOUT THE

DISCUSSION IN THIS ACTIVITY.

WE WILL WORK ON THE SPECIFICS OF WRITING ONE OF THESE SOON!

PublicationDon’t forget… publishing results

1. Allows other scientists to build on what has already been done.

2. Provides the opportunity for scientists to check other scientists’ work (called peer-review) ensuring the validity of conclusions.

PublicationIf other scientists do not achieve the

same results, the initial experiment and its conclusion will be rendered faulty.

This is called the “self-correcting” quality of science… which means it

corrects itself (eventually) as part of the normal process.

(add this term to your list)