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Chapter 6

Chapter 6

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University of Toronto Scarborough
David Nussbaum

Chapter 6 True Experiments – Single Factorial Designs  Cause and effect, it dictates an experimental design known simply as a true experiment  An experiment is a controlled investigation in which one or more variables are manipulated  In a true experiment, a researcher has complete control over the manipulation of the independent variable  Such experimental control allows for testing whether systematically varying the independent variable causes changes in the dependent variable The basics of experimentation  The defining feature of an experiment is the degree of control a researcher exercises over all aspects of the study design  The researcher directly controls the formation of groups, controls what levels of the independent variables are to be manipulated, how the experiment is conducted by holding constant certain aspects, such as the time, place, and setting of the study Experimental Control  Such experimental control is thus built in to a study’s design  A researcher can also exercise statistical control in analyzing data  A reasearcher exercises control in a study by holding constant as many extraneous variables as possible  If a variable is held constant, it cannot account for the results of the experiment  That is, the statistically significant results of a study establishing a relationship between changes in the independent variable and performance on a dependent variable cannot be confounded by a third variable Randomization  Another technique to control for the unwanted effects of a variable is known as randomization  For example, a memory test where you learn a list of words that vary in terms of visual imagery  Here our independent variable is visual imagery of words  Words presented at the beginning or at the end of a list tend to be remembered better than words in the middle of the list  The position of the word on a list, whether at the beginning, known as the primary effect or at the end known as the recency effect can confound the results of an experiment that may be examining visual imagery and recall  The process of random assignment ensures that any extraneous influence is just as likely to affect one group as the other group  In many psych studies, however an experimenter cannot use random assignment to create experimental and control groups  In these instances, the levels of the independent variable are selected rather than directly manipulated by the experimenter, that is, participants are selected and grouped after the fact or ex post facto, on the basis of a particular characteristic , such as gender, race, age, or diagnoses  Under these circumstances, the researcher conducts a quasiexperiment Independent Variables  An independent variable is something that a researcher manipulates in some way in order to determine its effects on a dependent variable  They are often referred to as factors  An experiment must have at least one independent variable that is manipulated by the experimenter, otherwise it would not be an experiment  An experiment with one independent variable is called a single-factor experiment  Experiments with more than one independent are often referred to as multifactorial designs  The term condition simply means a particular way in which participants are treated  For example, an experimenter may compare two groups of participants—those who receive treatment versus those who receive no treatment  Here condition labelled ‘group’ and treatment can be used interchangeably to refer to the independent variable in terms of various kinds of conditions or treatments, but not as groups, as there is only one group of participants that receives all conditions  The group of participants who received the level of independent variable that is the focus of the study is called the experimental group or treatment group  the group of participants who received the computer based active training is called the comparison group  the group that received no training is called the control group  do not confuse a control group with a control variable, which is defined as a variable that is held constant  the control group provides a means to determine changes that might occur naturally in the absence of treatment  the control group thus provides a baseline against which the treatment or experimental group can be compared Dependent Variables  all true experiments have as the dependent variable a posttest—that is, measurement of the outcome in both groups after the experimental group has received the treatment  many randomized experiments also have pretests that measure the dependent variable prior to the experimental intervention  however, using identical pre and post-tests is to be avoided, especially if performance is likely to be influenced by practice in taking the same measure twice  this creates problems because a researcher wants to ensure that any observed pre and post-tests changes are attributable to the independent variable and not to practice effects  here the use of pretest and post-test measures that have alternate forms would be wise, you can check to see if your pretest and post-test measures have good alternate forms reliability Control Variables  Some researchers’ control variables are others’ independent variables. This means that a control variable is a potential independent variable that is held constant throughout the experiment because its influence is thought to be extraneous, or unimportant to the goal of the study Research Design Alternatives  with a between-subjects design, independent groups of participants receive the different levels on the independent variable  by contrast, a within-subjects design is one in which all research participants receive all levels of the independent variable Between-subjects Design  with 2 levels of an independent variable, some participants receive one level and others receive the other level of the independent variable, the same participants never receive all levels of the independent variable  a different group of participants must be assigned to each level of the independent variable  for this reason, between subjects designs require a large number of people to be enrolled in a study  in these studies, a primary concern of the researcher is to ensure that there is no chance of one treatment contaminating the other (this design is best at doing just that) Randomization and matching  random assignment helps to ensure that groups are similar prior to the manipulation of the independent variable  people have different psychologies, and these individual differences are equalized or neutralized when researchers use random assignment to form experimental and control groups  a pre
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