LAB 4 Experimental Design

Transcription

1 LAB 4 Experimental Design Generally speaking, the research design that is used and the properties of the variables combine to determine what statistical tests we use to analyze the data and draw our conclusions. The figure below depicts a series of questions about these features to help one decide what statistic is appropriate. Today's lab will focus on the top part of this figure that involves experimental designs. Part I: Experiment basics The Experimental Method A method of determining whether independent variables are related to dependent variables (we will focus on designs with a single IV and a single DV). The researcher systematically manipulates the environment (manipulates the independent variable[s] and controls all other relevant variables either by randomization or by direct experimental control) to observe the effect of this manipulation on behavior. The method allows a causal inference to be made: Any change in the dependent variable was caused by the manipulation of the independent variable Designing an Experiment At the heart of an experiment is a comparison between two (or more) conditions. In other words, you (the experimenter) will always be comparing at least two things. This may include comparing your sample with a known population, or two (or more) different samples (groups) against each other, or even multiple scores

2 within a single sample of individuals. Generally the process involves a number of steps: identification of your research questions identifying your variables of interest specifying your hypotheses (how are the variables related to one another) selecting a research design collecting your data, analyzing your data drawing conclusions from your data about your hypotheses. Let's consider an example: Consider the steps that one may go through trying to design an experiment to test the following claim: Claim: Chocolate-covered peanuts enhance memory a) Construct a formal hypothesis e.g., chocolate-covered peanuts improve recall scores from a list of unrelated nouns. b) Identify the independent and dependent variables. IV: consumption of chocolate-covered peanuts DV: a measure of memory performance c) How to manipulate the IV the presence or absence of m&m's. How about manipulating the quantity of m&m's d) Do we need a control group, a placebo? Any other control variables? e) Identify the how we'll measure the DV. f) Any operational definitions or constructs needed? g) Are there any subject relevant variables (use randomization and matching) Are the effects the same for all sexes? Ages? Majors? h) Situation relevant variables (test conditions, experimenter behavior, timing) Identifying your Variables e.g., the list of nouns, how fast given Conceptual vs. Operational level of analysis Conceptual variables are often abstract theoretical entities which have corresponding operational variables in the experiment (concrete so that they can be measured or manipulated) Different subclasses of variables i) Independent variables - these are the variables that are manipulated by the experimenter, results in the different conditions in an experiment Event/Stimulus manipulations - manipulate characteristics of the stimuli, context, etc.

3 Instructional manipulations - different groups are given different instructions Subject manipulations - there are (pre-existing mostly) differences between the subjects in the different conditions (may lead to a quasi-experimental design) j) Dependent variables - these are the variables that are measured by the experimenter, they are "dependent" on the independent variables (if there is a relationship between the IV and DV as the hypothesis predicts). k) Control (extraneous) variables Holding things constant - Controls for excessive random variability l) Random variables - can vary freely from subject to subject m) Confounding variables (or lurking variables) Other variables, that haven't been accounted for (manipulated, measured, randomized, controlled) that co-vary with the IV and can impact changes in the dependent variable(s) - control or random variables can also be confounding variables if they affect the difference in results for the groups OPEN A BLANK MSWord DOCUMENT, LABEL IT LAB 4 ASSIGNMENT with your name and Univ ID # at the top, and type up your answers to the following questions. You should then upload your document when you finish with the entire lab. 1. For the following descriptions, identify the different variables. a) A doctor suggests to you that the drug triazolam (marketed as Halcion ) helps "reset the body's clock" for people experiencing jet lag. You conduct an experiment to test this hypothesis. Fifty people are flown from Atlanta to Tokyo. During the flight, half the flyers are given a dose of triazolam. The other half are given a harmless sugar pill. Six hours later, all subjects are asked to rate how sleepy and disoriented they feel. Identify the independent (or explanatory) variable(s). Identify the dependent (or response) variable(s). b) Two new drugs, ZZT and Glomulin, are believed to slow the progression of AIDS-related syndrome. To determine their effectiveness, the Food and Drug Administration authorizes a clinical test. Thirty HIV-infected patients are given each drug. In both groups, 20 patients show no signs of AIDs-related complications a year later. The researchers conclude that the drugs are equally effective in combating the symptoms of AIDS. Identify the independent (or explanatory) variable(s). Identify the dependent (or response) variable(s). 2. A researcher wants to know if brighter lights make factory workers more productive. Workers in a factory are randomly assigned to 2 groups. One is moved to a new factory next door where the factory lights are brighter. The other group stays behind in the old factory. The productivity of the 2 groups is compared. a. Identify the independent (or explanatory) variable(s). b. Identify the dependent (or response) variable(s). c. What is a plausible confounding variable? 3. A researcher wants to know if it helps patients if their therapists disclose personal information about themselves. Participants are randomly assigned to 1 of 2 groups. One group has therapists who previously have indicated that they tend to disclose a lot about themselves in therapy. The other group has therapists who previously have indicated that they rarely disclose personal information in therapy. a) Identify the independent (or explanatory) variable(s).

4 b) Identify the dependent (or response) variable(s). c) What is a plausible confounding variable? Selecting your experimental designs. As mentioned above your experiment will involve a comparison between at least two groups. But there is more to an experimental design than the creation of two groups. There are a number of different ways to create your two (or more) groups. a) Levels of your independent variables - there needs to be at least two different values (levels) of your IV. You may handle the different levels of your IV in two different ways a. Independent (between) and Related ("within" or "repeated" and "matched") variables i. independent samples - you manipulate your indpendent variables across separate groups of people (so each level of your IV is given to a different group/sample of individuals) ii. related samples - have everybody get all the different levels of your IV. Below is a decision tree that lists some common experimental designs. If you open the chart by clicking on the button, you will see a larger version of the chart with the kind of statistical analysis test that is used to analyze each one. Later in the course we'll be learning about how to conduct (some of) these statistical tests.

5 How do we use the decision tree? An example Suppose that you (a stats instructor) are interested in how well your lecture on experimental design worked. So you decide to test your students before and after the lecture. Both tests are designed to measure the students' knowledge of experimental design issues. What kind of experimental design is this? Go through the questions in the tree. b) How many groups (samples) of people do you test? 1 c) How many scores (pieces of data) do you collect from each person? 2 This leads you to a "within-subjects design." You've got one group, with two scores (pre-test scores and post-test scores) from each person (so the scores are "related" to each other by virtue of being from the same people). The IV here (2 levels: before lecture and after lecture) is being manipulated as a "within groups" variable

6 Part II: Group Project (form a group at the table nearest you). Your group's task is to take the issue below assigned to your group by the lab instructor and design an experiment (or series of experiments if you deem it necessary) to examine it. 1) Does watching violence on TV cause violent behavior? 2) Does playing video games improve hand-eye coordination in other tasks? 3) Does smoking cause lung cancer? 4) Does studying with background music improve test scores? 5) Does living in a large city decrease helping behaviors? 6) Does color affect mood? 7) Does caffeine affect work productivity? Be sure to include information about: what are your variables, how will you manipulate your independent variables, how will you measure your dependent variables, what control conditions do you need, who are your participants, etc? Make sure that you give enough thought and detailed discussion to all of these issues. 4. Individually, type up a description of the experiment that your group designed (including the information mentioned above). Then spend some time evaluating the experiment that your group designed. You should consider the adequacy of: a. the way the dependent variable(s) is/are measured. b. the way the independent variable(s) is/are manipulated. c. are there enough appropriate control conditions? d. what, if any, threats are there to internal and external validity? e. can you think of any potential confounds? f. using the decision tree, what design would you use to describe your experiment? (Optional If you have time after your group discussion!) Part III. Experimental designs and SPSS data format (looking ahead a bit). You do not need to write any answers for this part of the lab; rather, this section introduces you to some SPSS information that we will work on in the next few weeks this is just a preview FYI The design that you use for your experiment (or more generally your research design) will determine, in part, how you set up your SPSS datafile. This is another example of the importance of needing to know the context of your data. For some designs, the data for different levels of the Independent Variable (IV) will be set up in separate columns. This allows SPSS to be associate particular data points with matched data points. For example below you see a screenshot of a datafile that compares a sample of individuals who were in a study that compared their jumping performance before (Jump1) and after (Jump2) an experimental manipulation. For this analysis, SPSS needs to "know" which jumps correspond to which individuals because we will compare, on a person-by-person basis, their first jump with their second jump. The way SPSS "knows" this is by putting these data points in the same rows. Notice the data points are the measurements of the Dependent Variable

7 (DV) in the different levels of the IV. For other designs your Independent variable will be in a single column, with values in that column specifing the levels of that variable. These are situations where the computations don't rely on pairwise comparisons of specific data points. In these designs, the data points for the DV are in a separate column. Computationally, all SPSS needs to "know" is which scores are in which levels of the IV. There is no need to specifically match specific datapoints together. Below is a screen shot (from an older version of SPSS, but the details of the point here are the same). Math_Scores are the scores for the DV. Study Group are the levels of the IV.

8 .