Lecture - 4 - April 6th

# Lecture - 4 - April 6th - STATISTICS IN PSYCHOLOGICAL...

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STATISTICS IN PSYCHOLOGICAL RESEARCH 1. Descriptive Statistics Correlation A correlation exists when two variables are related to each other. A correlation may either be: High School GPA College GPA Absences Test Scores 1 2.1 2.3 2 2.4 2.2 3 2.7 2.8 4 2.9 3 5 3.2 3.5 6 3.3 3.4 7 3.6 3.6 8 3.9 4 Student 1 10 25 2 8 45 3 7 65 4 6 67 5 5 70 6 4 80 7 3 85 8 1 99 Student Positive Correlation Negative Correlation

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STATISTICS IN PSYCHOLOGICAL RESEARCH A Positive Correlation A Negative Correlation High School GPA College GPA High School GPA College GPA Absences Exam Scores Absences Exam Scores 2 2.5 3 3.5 4 2 2.5 3 3.5 4 0 25 50 75 100 0 2 4 6 8 10 College GPA Exam Scores Absences High School GPA Scatterplot Scatterplot
STATISTICS IN PSYCHOLOGICAL RESEARCH 1. Descriptive Statistics Correlation Coe cient A correlation coe cient ( r ) is a numerical index of the degree of relationship between two variables. It is a number between - 1 and 1. The sign + or - indicates directio n of the relationship The number indicates strength of the relationship.

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STATISTICS IN PSYCHOLOGICAL RESEARCH 1. Descriptive Statistics Correlation and Causation Correlations allow us to predict the value of one variable based on knowledge of the other variable. What correlations allow us to do: 2 2.5 3 3.5 4 2 2.5 3 3.5 4 0 25 50 75 100 0 2 4 6 8 10 College GPA Exam Scores Absences High School GPA
STATISTICS IN PSYCHOLOGICAL RESEARCH 1. Descriptive Statistics Correlation and Causation What correlations do not allow us to do: Correlations do not tell us whether a cause - e f ect relationship exists between two variables. For example: Does low self - esteem cause depression? Correlation Does Not Imply Causation!

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STATISTICS IN PSYCHOLOGICAL RESEARCH 2.Inferential Statistics What is the likelihood that our observed di f erence may have simply come about by chance? If the likelihood that our observed di f erence may have simply come about by chance is very low ( p - value < .05 ) , then we say it is signifcan t . Marijuana and Driving Performance: Results Marijuana Group Placebo Group Mean No. of Collisions 3.6 2.1 Inferential statistical procedures allow us to answer this question.
STATISTICS IN PSYCHOLOGICAL RESEARCH 2.Inferential Statistics A statistical di f erence does not mean a practical di f erence! But be careful:

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## This note was uploaded on 10/11/2011 for the course PSYC 1 taught by Professor Stuartantsis during the Spring '08 term at UCSD.

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Lecture - 4 - April 6th - STATISTICS IN PSYCHOLOGICAL...

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