Experimental Design independent and dependent variables & writing a hypothesis statement

What is the effect of a lack of sleep on weight gain by an individual?

What is the effect of a lack of sleep on weight gain by an individual? Before we can perform an experiment designed to answer this question, we must determine the independent and dependent variable. For an experiment to have valid results, there can be only one independent variable. THIS IS IMPORTANT!

Independent Variable The independent variable is the variable that you, as the scientist, will vary to observe the effect The independent variable is sometimes called the manipulated variable because it is the variable the scientist manipulates.

Dependent Variable The dependent variable is the variable that you, as the scientist, will measure and record – it’s the data of the experiment The dependent variable is sometimes called the “responding” variable because it is the variable that responds to the independent (or manipulated) variable

What is the effect of a lack of sleep on weight gain of an individual? To determine the independent and dependent variable, identify the two things that will (might) change: lack of sleep weight of an individual

What is the effect of a lack of sleep on weight gain of an individual? Once you know the two things that are (or might be) changing, arrange them in a question like this: In this experiment, does the lack of sleep depend on weight gain? OR Does weight gain depend on lack of sleep? The variable that depends on the other is the dependent variable.

What is the effect of a lack of sleep on weight gain of an individual? A lack of sleep depends on weight gain? OR that weight gain depends on a lack of sleep? Weight gain depends on the lack of sleep so weight gain is the dependent variable.

About This Process It’s important to understand that you don’t yet have to know if weight gain depends on a lack of sleep – you just need to identify which variable you will be manipulating and which variable will respond to your manipulations.

Practice Identifying Variables What effect does being raised on a fish farm have on a salmon’s length? What two factors are being studied? salmon’s length raised on a fish farm

Practice Identifying Variables What effect does being raised on a fish farm have on a salmon’s length? Does being raised on a fish farm depend on the salmon’s length? OR Does the length of the salmon depend on being raised on a fish farm?

Practice Identifying Variables The length of the salmon depends on being raised on a fish farm Independent Variable? raised on fish farm Dependent Variable? length of salmon

Practice On the following slides, write down the research question in your notes and then discuss with your partner what you believe the two factors that will (might) change are. After you agree on the two factors, see if you can agree on which one is the independent variable and which one is the dependent variable.

Do tomato plants raised with music grow more tomatoes? Two factors being studied? presence of music number of tomatoes grown

Do tomato plants raised with music grow more tomatoes? independent variable? dependent variable? presence of music number of tomatoes

Does regular exercise decrease the number of colds a person becomes infected with? Two factors being studied? frequency of exercise number of infections with cold virus

Does regular exercise decrease the number of colds a person becomes infected with? independent variable? dependent variable? exercise number of colds over a specified time period

Do people that use cell phones while driving have more automobile accidents? Two factors being studied? use of cell phones while driving number of automobile accidents involved in

Do people that use cell phones while driving have more automobile accidents? independent variable? dependent variable? use of cell phones while driving number of auto accidents involved in

Writing a Hypothesis Once you know the independent and dependent variables, writing a hypothesis statement is easy! A hypothesis simply combines the independent and dependent variables into a prediction statement.

Writing a Hypothesis Research Question: What effect does salt have on the boiling point of water? Independent Variable: adding salt Dependent Variable: boiling point of water

Writing a Hypothesis IV: adding salt to H2O DV: boiling temp A good hypothesis statement is an “if” “then” statement. If salt is added to water, then the boiling point of water will increase.

Writing a Hypothesis If salt is added to water, then the boiling point of water will increase. The independent variable is the “if” part of the hypothesis and the dependent variable is the “then” part.

A Hypothesis IS a statement that predicts something testable an “if” “then” statement

A Hypothesis is NOT: a question a statement that has words in it like maybe probably might

What a Hypothesis Might Have A hypothesis might also have a “because” section In the prior example, the “because” would sound like this: If salt is added to water, then the boiling point will increase because salt is an ionic compound and ionic compounds are known to elevate boiling points.

Hypothesis Practice What is the effect of wing size on the falling speed of a rotocopter? What are the two factors? wing size falling speed Which factor is the independent variable? Dependent variable? ind v: wing size dep v: falling speed

Hypothesis Practice ind v: wing size dep v: falling speed If the wing size is increased, then the falling speed of the rotocopter will decrease. or If the wing size is increased, then the falling speed of the rotocopter will decrease because increased wing size will result in increased air resistance.

Hypothesis Practice What effect does sandpaper have on a block of wood’s sliding velocity? Two factors? sandpaper sliding velocity Ind V and Dep V? IV: sandpaper DV: sliding velocity

Hypothesis Practice IV: sandpaper DV: sliding velocity If a block of wood is pulled on a sandpaper surface instead of a wooden surface, then the sliding velocity will decrease. or If a block of wood is pulled on a sandpaper surface instead of a wooden surface, then the sliding velocity will decrease because the force of friction of the sandpaper will be greater.