1. Section 2.4: Simple Comparative Experiments

2. Experiment – A planned intervention undertaken to observe the effects of one or more explanatory variables, often called factors on a response variable. Experimental Condition (treatment) – Any particular combination of values for the explanatory variables Design – The overall plan for conducting an experiment

3. Example We want to see if exam scores are affected by room temperature. Suppose we have four classes of AP Statistics. Two rooms have temperatures of 65 degrees and two have temperatures of 75 degrees. Exam scores for the 65 degree room are much higher. Can we conclude that the increased temperature resulted in lower exam scores?

4. Example continued… No. Factors that can affect our outcome: –Different time of day –Different instructors –Different textbooks –More homework –Abilities of students

5. Extraneous Factor – Is not of interest in the current study but is thought to affect the response variable. A researcher can control some extraneous factors. In previous example the textbook could be an extraneous solution. If you choose to control this factor, you can require all sections to use same textbook.

6. If one instructor taught both 65 degrees session and the other taught both 75 degree sessions we would be unable to distinguish the effect of temperature from the effect of the instructor. These two factors are said to be confounded. Confounded – Two factors whose effects on the response variable cannot be distinguished from one another.

7. Key Concepts in Experimental Design Randomization – Random assignment (of subjects to treatments or of treatments to trials) to ensure that the experiment does not systematically favor one experimental condition over another Blocking – Using extraneous factors to create groups (blocks) that are similar. All experimental conditions are then tried in each block. Distant Control – Holding extraneous factors constant so that their effects are not confounded with those of the experimental conditions Replication – Ensuring that there is an adequate number of observations for each experimental condition

8. Before proceeding with an experiment, you should be able to answer each of the following 10 questions 1.What is the research questions that data from the experiment will be used to answer? 2.What is the response variable? 3.How will the values of the response variable be determined? 4.What are the factors (explanatory variables) for the experiment? 5.For each factor, how many different values are there, and what are these values? 6.What are the treatments for the experiment?

9. 7. What extraneous variables might influence the response? 8. How does the design incorporate random assignment of subjects to treatments or random assignment of treatments to trials? 9. For each extraneous variable listed in question 7, how does the design protect against its potential influence on the response through blocking, direct control, or randomization? 10. Will you be able to answer the research question using the data collected in this experiment?

10. Activity: An Experiment to Test for the Stroop Effect