Psyc_315_-Winter_2010_-_Class_22

Psyc_315_-Winter_2010_-_Class_22 - Independent Samples...

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Independent Samples t-Test Comparing the Means of Two Populations Chapter 13 2 Dependent vs Independent • When a researcher uses the same individuals in both groups, as in a before-after comparison, they are said to be Dependent samples. • When none of the observations in one group is in any way related to observations in the other group then they are said to be Independent samples. – Very common in psychological research (experiment vs control). • E.g. – Males vs Females scores in an Statistics Exam. – Male vs Female salary in same job. – GPA in Psychology students from Concordia and McGill. 3 Sampling Populations • Because we have two samples, how we create our samples is important. • Therefore, we must make sure our subjects are randomly assigned to a treatment. • The random assignment of subjects to treatment conditions is called randomization .
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4 Role of Randomization • “Randomization is a method for dividing an available pool of subjects into two or more groups. It refers to any set of procedures that allows chance to determine who is included in what group.” • Randomization provides experimental control over extraneous factors that can bias the result. • The “why” of one’s results is much clearer when subjects have been randomly assigned to treatment conditions. • The investigator can be much more confident that the manipulated factor is the only factor that explains any group difference subsequently obtained. 5 Role of Randomization • Sometimes randomization is not possible: – Suppose you wish to compare groups differing on such characteristics as sex, political party, or ethnicity. – In this case, you cannot randomly assign subjects to the “treatment condition” male or female, Liberal or Conservative, and so on. – You lose a considerable degree of control of extraneous factors, and determining the “why” of results is not easy. – The comparison is confounded with uncontrolled, extraneous factors. – Must be careful in interpreting the results of such a study. 6 The Sampling Distribution of Differences Between Means • If you were to repeat an experiment many, many times with a new random selection of subjects and obtain a difference score between means each time, this will form a sampling distribution of differences between means . • Properties of the sampling distribution of differences between means: – This distribution is defined by its mean, standard deviation, and shape. – The mean of all the differences will be zero because the mean of the positive differences will be balanced by the negative differences.
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7 The Sampling Distribution of Differences Between Means • The standard deviation of the sampling distribution of differences between means is known as the standard error of the difference between means . • The standard error of the difference between means is
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Psyc_315_-Winter_2010_-_Class_22 - Independent Samples...

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