Writing HypothesesDefinition Variables Formatting Hypotheses Linking Conclusions to Hypotheses

DefinitionA hypothesis is a tentative statement that proposes a possible explanation to some phenomenon or event. A useful hypothesis is a testable statement which may include a prediction.

A hypotheses should not be confused with a theory. Theories are general explanations based on a large amount of data. For example, the theory of evolution applies to all living things and is based on wide range of observations. However, there are many things about evolution that are not fully understood such as gaps in the fossil record. Many hypotheses have been proposed and tested.

DefinitionThe key word is testable. That is, you will perform a test of how two variables might be related. This is when you are doing a real experiment. You are testing variables.

VariablesA variable is any factor, trait, or condition that can exist in differing amounts or types. An experiment usually has two kinds of variables: independent and dependent.

VariablesThe independent variable is the one that is changed by the scientist. A good experiment has only one independent variable. As the scientist changes the independent variable, he or she observes what happens. The scientist focuses his or her observations on the dependent variable to see how it responds to the change made to the independent variable. The new value of the dependent variable is caused by and depends on the value of the independent variable.

VariablesExample: If you open a faucet (the independent variable), the quantity of water flowing (dependent variable) changes in response--you observe that the water flow increases. Independent Variable (What I change) Water faucet opening (closed, half open, fully open) Dependent Variable (What I observe) Amount of water flowing measured in liters per minute

Practice

Identify the dependent and the independent variables in each of the following:

1. Chocolate may cause pimplesDependent Variable: Pimples

Independent Variable: Chocolate

2. Salt in soil may affect plant growth.Dependent Variable: plant growth

Independent Variable: salt in soil

Practice

3. Is plant growth affected by the color of the light? Dependent Variable: Plant Growth Independent Variable: light 4. Bacterial growth may be affected by temperature. Dependent Variable: Bacterial growth Independent Variable: temperature

Practice

5. If skin cancer is related to ultraviolet light , then people with a high exposure to ultraviolet light will have a higher frequency of skin cancer. Dependent Variable: skin cancer Independent Variable: ultraviolet light

Practice

6. Temperature may cause leaves to change color. Dependent Variable: leaf colour change Independent Variable: temperature

Formatting HypothesisHypotheses take three formats: A question, "Does temperature affect fermentation? A conditional statement, "Temperature may affect fermentation. An If, then statement, "If fermentation rate is related to temperature, then increasing the temperature will increase gas production. (formalized hypothesis)

Formalized HypothesisA caution is necessary at this point.

Beware! Not all "if-then statements are hypotheses. For example, "If you warm yeast, then more gas will be produced." This is a simple prediction, not a hypothesis! The problem with this statement is that there is no proposition to test. What is related to what? Is temperature a variable? Is yeast a variable? Is gas production a variable?

Formalized HypothesisThe use of the phrase "is related" is intentional in these examples. Other phrases such as "is affected" will work here, but I prefer "is related" because it reminds students that we are investigating relationships, not just cause and effect events. Students get into less trouble if they stick to this phrase.

Formalized HypothesisIn a formalized hypothesis, a tentative relationship is stated. For example, if the frequency of winning (dependent) is related to frequency of buying lottery tickets (independent). "Then" is followed by a prediction of what will happen if you increase or decrease the frequency of buying lottery tickets. If you always ask yourself that if one thing is related to another, then you should be able to test it.

A Very Important NoteA hypothesis is still valid even when results are the opposite of what is predicted because it will still shed light on the true nature of the relationship being tested. This lowers the risks of being wrong. For example, "If the period of a pendulum is related to its length, then the longer the pendulum the shorter the period." Actually the results show just the opposite is true.

A Very Important NoteHypotheses that predict unrelated variables are also useful. For example, "If the period of a pendulum is related to its mass, then increasing the mass will increase the period." As it turns out, the mass has no effect at all, as the student will discover while testing this hypothesis.

Linking Conclusions to HypothesesStudents are to restate what is the relationship, describe it, and tell how she/he knows it is so. The key to good conclusions is to establish standards for what is expected and then model it. A solid conclusion should always relate back to the original hypothesis, describe how the relationship works, and relate how the data supports this conclusion. Conclusions are often inferences drawn from what observations and data imply.