Name______________________________________ Period:______________________________________
Objective: I will be able to identify flaws in experiments and correct them.
Danger: Common Flaws in experiments
There are 3 common flaws (mistakes) made in experiments. We will look at each common mistake and examples. You
will practice identifying the flaws in some example experiments and explaining how to fix the
experiment.
Flaw #1: More than 1 independent variable
Every experiment in science should have only 1 independent variable. If there is more than one
independent variable, you cannot tell if the change in your dependent variable was because of
which variable.
Example: Jose wants to know if the type of skateboard you use affects how well you can skate. Jose decides to test 5
different skateboards. He thinks his shoes might also change how well he skates, so he wears different shoes each time
he skateboards too. Jose finds that skateboard number 1 was the best.
What is wrong with the experiment? Jose had 2 independent variables. He changed his shoes and his skateboard.
How can we fix his experiment? Jose should wear the same shoes every time he skates and only change the skateboard
that he uses.
Flaw #2: Too small of a sample size
Sample size is the number of subjects (items) that you test on in your experiment. In order to
get reliable results, experiments should have large sample sizes. For example, when the FDA
tests medicines, they test it on hundreds of people (sample size=300), not just 2 (sample size
=2).
Example: Carlos is interested to see if watering a plant with coffee affects how tall it grows.
Carlos has 2 plants. He waters one with 150mL of water and the other plant with 150mL of
coffee each day. Carlos does this every day for 3 weeks and records how tall the plants are each
day.
What is wrong with the experiment? Carlos only had a sample size of 2 plants.
How can we fix his experiment? Carlos should test his experiment on more plants. Maybe Carlos should use 20 plants in
his experiment. He should water 10 if them with water and the other 10 with coffee.
Flaw #3: Not enough trials
Trials are the number of times you repeat and experiment. In order to have reliable results, experiments should be done
more than 1 time.
Example: Ms. Dixon is interested to see if completing a warm-up at the beginning of class affects students grades. Ms.
Dixon has a warm-up on the board on Monday, and no warm-up on the board on Tuesday. Ms. Dixon collects the work
from each day. Ms. Dixon sees that students scored higher on Monday and decides that a warm-up does in fact affect
students’ grades.
What is wrong with the experiment? Ms. Dixon only had 1 trial.
How can we fix his experiment? Ms. Dixon should increase her number of trials. She should perform the experiment for
3 weeks.
Practice! Practice! Practice!
In each problem below, identify the flaw in the experiment and explain how we can fix the experiment.
1. Cindy wants to know if running gives her more energy or if watching TV gives her more energy. Cindy decides to
test this. For 1 week Cindy eats only vegetables for snack and runs 4 miles after school. Cindy records her energy
level. The next week Cindy eats only hot Cheetos for snack and watches TV for 3 hours after school.
What is wrong with the experiment? _______________________________________________
How can we fix this experiment? ______________________________________________
_________________________________________________________________________
_________________________________________________________________________
2. Ms. Foss wants to know if the temperature in classrooms affects how well students do on
their quizzes. Ms. Foss decides to record the temperature in each of the classrooms at
MLK and record students’ quiz scores. Ms. Foss concludes that the room temperature
does change students’ test scores.
What is wrong with the experiment? __________________________________________
How can we fix this experiment?
_________________________________________________________________________
_________________________________________________________________________
3. Mrs. Welk wants to know if the size of the dog affects how fast it dries after it gets a bath.
Mrs. Welk decides to wash her dog Olive, who is 8 pounds and time how long it takes her to dry. Mrs. Welk then
asks her sister to give her dog, Tank, a bath and time how long it takes him to dry. Tank weighs 70 pounds.
What is wrong with the experiment? __________________________________________
How can we fix this experiment? _______________________________________________________________
___________________________________________________________________________________________
4. Your Grandpa Henry says that his feet can predict the weather. He says that whenever his feet are sore in the
morning, it is going to rain. You’re not sure whether to believe your Grandpa Henry or not, so you plan an
experiment. Tomorrow morning, you will call and ask your Grandpa if his feet are sore. At the end of the day,
you will check and see if it rained. This will prove whether Grandpa Henry really can predict the weather.
What is wrong with the experiment? __________________________________________
How can we fix this experiment? _______________________________________________________________
___________________________________________________________________________________________
5. You want to know who can jump higher – World History teachers or Science teachers. So, you decide to test Mr.
Taylor and Ms. Dixon. You have them each jump and touch as high as they can on the wall. You measure the
height they touched, and the person who touched highest is the best jumper… right?
What is wrong with the experiment? __________________________________________
How can we fix this experiment? _______________________________________________________________
___________________________________________________________________________________________
More Practice!
Fill in the blanks below. You may want to use the first page of your packet for help!
1. An experiment should only have __________ independent variable.
2. The ___________ ______________ is the number of people tested in an experiment.
3. ____________ are the number of times you complete an experiment.
4. ________________ is a prediction or an educated guess.
5. The ______________ variable is the variable you observe or measure.
6. The ________________ variable is what you change in an experiment.
7. A hypothesis should be written in and “_____, _____________” statement.
8. The more ____________ an experiment has, the more reliable the results are.
Putting it together
In the problems below, you will need to identify the independent, the dependent and the control variables, write a
hypothesis and explain if there is a flaw and how to fix it.
1. Principal Smith wants to know if teachers on the second floor are more fit. Mr. Smith decides to test this by
giving 2 teachers on the 1st floor a fitness test and 2 teachers on the 2nd floor a fitness test.
Independent Variable:_______________________________________
Dependent variable: _________________________________________
Control variables:________________________________________________________________
Hypothesis:
___________________________________________________________________________________________
___________________________________________________________________________________________
Flaw: _________________________________________________________________________
How to fix Mr. Smith’s experiment:
___________________________________________________________________________________________
___________________________________________________________________________________________
2. Here’s the experiment: A student plants four seeds – one pea, one bean, one acorn, and one watermelon seed
– in four identical small pots, in different positions. Each pot contains the same kind of soil in equal amounts.
She plans to grow the plants in the same room under the same lights. She will water them the same amount
every day and measure their growth for two weeks.
Seed type
Pea
Bean
Acorn
Watermelon
1. Which of the following is a controlled variable in this experiment? There may be more than one correct answer –
circle all correct choices.
A. The size of the plant
C. The type of seed
B. The position of the seed in the pot
D. The amount of water she uses
2. What mistake is she making in this experiment?
A. No dependent variable
D. Too many independent variables
B. Too many controlled variables
E. Too many independent variables
C. Scientific question is not testable
I notice….
I wonder…
Possible hypothesis
_______________________________________________________________________________________________
I notice….
I wonder…
Possible hypothesis
_______________________________________________________________________________________________
I notice….
I wonder…
Possible hypothesis
_______________________________________________________________________________________________
I notice….
I wonder…
Possible hypothesis