27 January EPSY 640 Spring Copyright Robert J. Hall, Ph.D. 1. Independent Dependent Control/Extraneous Intervening.

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1 Variables Independent Dependent Control/Extraneous Intervening Robert J. Hall Question A recent report concludes that rats given vitamin supplements have better maze-learning scores than rats on a regular diet. For this study the independent variable is a. the set of rats b. maze-learning c. the type of diet d. the differences among the rats Independent & Dependent Variables Regular Maze Learning Score Population of Rats Type of Diet Enhanced Maze Learning Score Type of diet - Independent variables classify subjects into groups. These groups may be based on some personal attribute or characteristic (e.g., gender, anxiety level) or they may be based on group membership (e.g., experimental/control). Type of diet classifies subjects into one of two groups and thus serves as an independent variable in this experiment. Maze Learning Score is the dependent variable; the score depends on (is a function of) the type of diet. Robert J. Hall, Ph.D. 1

2 Control (Extraneous) Variables A control variable is any variable which is held constant in a research study by observing only one of its levels. Control variables are used in research to neutralize the effects of variables which are not of central focus to the study but which may affect the observed behavior. For example, in the study of reading, sex might be controlled, since it is known to be related to reading performance. Intervening Variables An intervening variable is a hypothetical variable which is not observed directly but, rather, is inferred from the relationship between the independent variable and the dependent variable. An explanatory variable representing the foundations of theory. Hierarchical associative networks in memory - hypothetical variable To summarize, Researchers sometimes measure an entire population, calculate one or more parameters from their scores, and e.g., µ (mu) or σ (sigma) use them to describe the population. Robert J. Hall, Ph.D. 2

3 More commonly, Researchers are able to study only a sample from the population, calculate a statistic based on that sample, and X (X-bar) or s x use this statistic to draw inferences about the population. Because we cannot be sure that any particular sample of scores exactly reflects the population, we need to use inferential statistics to let us test hypotheses about the population and estimate its parameters. Estimates and other inferences based on statistical analyses are likely but not guaranteed to be correct. Unfortunately, there is no way to ensure the absolute accuracy of our answers. Thus, any inferences we make from our results have to be tempered by the understanding that they may or may not be correct. Our job as researchers is to maximize the probability associated with being correct while minimizing the probability of making incorrect conclusions. Robert J. Hall, Ph.D. 3

4 Constant: A characteristic or value that does not change and has the same value for everyone in a particular research study. Common examples: sex (all females), grade (all first graders), university attended by subjects (all attend TAMU - whoop!) Variable: A characteristic that takes on more than one value, which varies across individuals. 1. Qualitative variable: A variable in which the different scores on the variable represent differences in quality, character, or kind but not in amount. Also called a nominal-level variable. Common examples: sex, religion, birthplace 2. Quantitative variable: A variable for which different scores represent different amounts of the property being measured. Common examples: height, age, test scores, speed Continuous quantitative variable: a variable that can assume an infinite number of values - weight, age, time Discrete quantitative variable: a quantitative variable that can take on only a limited number of values - # of children in a classroom, # of cars owned Robert J. Hall, Ph.D. 4

5 A researcher studying the effects of environment on mood asks participants to sit alone in a waiting room for 15 minutes at the beginning of an experiment. Half of them are assigned to a room with dark blue walls, and the other half are assigned to a room with bright yellow walls. After 15 minutes in the waiting room, each participant is brought into the lab and given a moodassessment questionnaire. Questionnaire scores are then examined by the researcher. Last slide viewed Return to Study 1. What is the independent variable for this study? How many levels are there of the independent variable? What scale of measurement is associated with this variable? 2. What is the dependent variable for this study? What scale of measurement is associated with this variable? Is the measure discrete or continuous? Return to Study 3. What is (are) the extraneous (control) variable(s) in this study? 4. What is the intervening variable in this study? 5. Identify the population for this study. 6. Identify the sample for this study. 7. Give an example of a parameter. 8. Give an example of a statistic. Robert J. Hall, Ph.D. 5

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