Experimental Design

Homework Variables Worksheet due Monday. Mystery Powder Lab due Monday.

Objectives Identify three kinds of variables.

Variables Data is information. Measurements, observations, and facts are types of data. Variables are situations, objects, events, or states that are involved in the experiment. They can vary in an experiment, hence, variable.

Variables –Independent variable is the condition you believe will have an effect on the outcome. –Who can tell me about an experiment you’ve heard about or done?

Variables –Dependent variables depend on what the independent variable is, you do not directly manipulate them. What happens to them is the point of the test. –Going back to the same examples, let’s figure out dependent variables.

Variables –Control variables are constant across your experimental set-ups. You don’t want to test them. You want to make sure they don’t mess with your results. –What could be controlled in our examples?

Variables If I were to have a test in which I’m looking at the question, “What is the effect of exposure to different heat sources on plant height?” –What is my independent variable, the thing that doesn’t depend on any other variables? –What is/are my dependent variable/s, the thing/s whose values will depend on the independent variable? –What might some control variables be?

Variables Another one: “Which type of corrosive acid dissolves a bean plant most quickly?” –Independent variable? –Dependent variable/s? –Control variables?

Variables You have a homework worksheet to practice identifying different kinds of variables. Hint: I find that it helps to try to fit the experiment into the phrase “What is the effect of X on Y?” If you can do it, whatever went into the space X is your independent variable, space Y is your dependent variable.

Variables and Mystery Powders Today, you’ll design procedures and at least start running tests to try to figure out the contents of several mystery powders, based upon your background research. CHALLENGE: What will your independent, dependent, and control variables be?

Mystery Powders Check your Scientific Process questions 1-4 with me before you start collecting data. Make sure that everyone in your group is involved. Question before we begin: If science doesn’t always go step-by-step like this, why am I asking you to?

Fair Tests

Homework

Objectives Learn about what a “fair test” is Successfully design a fair test

Fair Tests A fair test is a test that provides accurate and reliable evidence to help answer the question you’ve asked. Suppose I want to know if higher temperatures help roses grow. If I plant some roses in Alaska and some roses in Siberia, will my results really help me answer my question? My data will reflect reality, but they may not be any good to me… What does a test have to be in order to be fair?

Fair Test How do you design a fair test? –Change only one independent variable at a time. –Think of all possible control variables, and keep them constant. –Use the right instruments to measure your results. –Arrange everything to make error unlikely.

Will It Float? Talk in your pairs, what are all the different things that you think could possibly affect whether something will float or sink?

Will It Float? What is or are the: –Independent variable/s? –Dependent variable/s?

Will It Float? Knowing the rules of a fair test, how do we determine if you’re right about those things? –Get together with two other tables nearby, get a whiteboard, and brainstorm what you could do (within the constraints of the classroom) to test these predictions.

Will It Float? What did you come up with? –Other groups, think carefully about these proposals, and make suggestions when they’re done. –We need to come up with a procedure that everybody agrees is a 100% fair test.

Will It Float? We’ll do this when we return tomorrow. –Who will be responsible for bringing in what materials?

Determining Volume Think of as many ways as you can to determine the volume of something. (Hint: you’ve already used at least 3 in this class)

Using Balances How to use a triple-beam balance How to use an electronic balance