Writing a Scientific Hypothesis and Assigning Variables
VARIABLES
Recall there are 3 sets of variables - the manipulated variable (always only 1), the
responding variable(s) (usually only 1 or 2) and the controlled variables (as many as possible)
 The manipulated variable (MV)- the variable you are testing or the variable you change
on purpose.
 The responding variable(s) (RV)-the variable(s) that may change as a result of the
manipulated variable.
 Controlled Variables (CV)- the variables kept constant or stay the same for both groups
because otherwise they might impact the responding variable.
FOR EACH QUESTION, WRITE DOWN THE VARIABLES. DO NOT ANSWER THE HYPOTHESIS
YET, WE WILL COME BACK TO THEM LATER
1. Do plants grow faster in soil which fertilizer has been added?
Manipulated Variable: _______________ Responding Variable: ______________
Control Variables (at least 3): __________________________________________
Hypothesis:__________________________________________________________
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
2. Does drinking caffeine before a math test increase test scores?
Manipulated Variable: _______________ Responding Variable: ______________
Control Variables (at least 3): __________________________________________
Hypothesis:__________________________________________________________
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
3. Does listening to cell phones cause damage to cells in your brain?
Manipulated Variable: _______________ Responding Variable: ______________
Control Variables (at least 3): __________________________________________
Hypothesis:__________________________________________________________
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
4. Does water temperature influence the amount of eggs laid by stream fish?
Manipulated Variable: _______________ Responding Variable: ______________
Control Variables (at least 3): __________________________________________
Hypothesis:__________________________________________________________
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
5. Are High School Athletes that get at least 8 hours of sleep more likely to play on the
varsity team than players who get less sleep?
Manipulated Variable: _______________ Responding Variable: ______________
Control Variables (at least 3): __________________________________________
Hypothesis:__________________________________________________________
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
6. Does playing video games increase your reaction time?
Manipulated Variable: _______________ Responding Variable: ______________
Control Variables (at least 3): __________________________________________
Hypothesis:__________________________________________________________
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
Hypothesis
Your hypothesis is your best guess about something, based on your experience or previous
knowledge. It doesn’t matter if your guess is correct or not. The scientific investigation will
determine whether your hypothesis is ‘right’ (supported) or ‘wrong’ (refuted/ not supported).
Guideline 1. Choose a format for your hypothesis:
A. If __ specific MV change___, then ____ specific RV change______ because …
-orB. ___ specific MV change____ causes* ___ specific RV change______ because…
*You can alternatively substitute ‘causes’ for many other words such as: ‘increases’,
‘decreases’, ‘leads to’, ‘results in’, etc.
Example Hypotheses:
 If you run 3 miles a day, then your blood pressure will be lower, because your heart
will be healthier.
 Reading at least thirty minutes a day causes your IQ to increase, because reading
stimulates brain activity.
The hypothesis and variables are connected
Writing your hypotheses following the guidelines above makes finding the variables easy! If
you choose the suggested formats (under guideline 1), then finding the manipulated and
responding variables should be easy!
A. If __(MV)_______, then _____(RV)_____ -orB. ___(MV)____ causes _____(RV)_____
Example:
If you take Vitamin C supplements, then it will decrease the length of your cold, because
Vitamin C is an essential element for your immune system to function, so increasing the
amount of Vitamin C in your body will help your immune system fight off the cold faster.
Manipulated Variable (MV): ______________________
Responding Variable (RV): _________________________
Controlled Variables (CV): all other factors that would be held constant or could otherwise
for affect the length of a cold, such as:
Guideline 2. Write your hypothesis as a statement (not a question).
Example:
Incorrect Format: Does taking vitamin C help you get over a cold?
Better Format: Taking vitamin C decreases the length of a cold, because...
Better Format: If you take Vitamin C it will decrease the length of your cold, because…
Guideline 3. Your hypothesis needs to be testable.
Make sure you are specific enough so that you could actually design an experiment to test
your hypothesis.
Example:
Incorrect Format: Eating a good breakfast increases your ability to learn.
Better Format: Eating a breakfast high in protein improves your memory (more specific)…
Guideline 4. Follow up your hypothesis with an explanation or a ‘because’ clause.
Your hypothesis should be based on your previous experience, knowledge or ideas. It should
be more than just a random guess. Using a ‘because clause’ after your hypothesis is a good
way to accomplish this.
Example: If you take Vitamin C supplements, then it will decrease the length of your cold,
because Vitamin C is an essential element for your immune system to function, so increasing
the amount of Vitamin C in your body will help your immune system fight off the cold faster.
Guideline 5: The hypothesis needs to predict the relationship between two variables.
Example: Does adding compost to sunflower plants make them grow taller?
Incorrect Format: If compost makes sunflower plants grow taller, then compost should be
used to help sunflower plants grow because tall plants are desired. Note- there is no
prediction about the effects of compost on sunflower growth.
Correct Format: If compost is added to sunflower plants, then the sunflowers will grow taller
because compost provides the nutrients need for plant growth.
NOW GO BACK TO THE EARLIER QUESTIONS AND USING THE GUIDELINES ABOVE WRITE
A HYPOTHESIS FOR EACH.