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Correlations. Captures how the value of one variable changes when the value of the other changes Use it when: Test the relationship between variables.

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Presentation on theme: "Correlations. Captures how the value of one variable changes when the value of the other changes Use it when: Test the relationship between variables."— Presentation transcript:

1 Correlations

2 Captures how the value of one variable changes when the value of the other changes Use it when: Test the relationship between variables Only two variables at a time

3 Ranges from -1 to +1 A Pearson correlation is based on continuous variables Important to remember this is a relationship for a group, not each person Reflects the amount of variability shared by two variables

4 r xy = n ΣXY - ΣX ΣY [n ΣX 2 – (ΣX) 2 ][n ΣY 2 - (ΣY) 2 ] r xy = correlation coefficient between x & y n = size of sample X = score on X variable Y = score on Y variable

5 A study of the relationship between years of post-high school education and income (in 1,000s) ParticipantsYrs EducationIncome

6 1. State hypotheses Null hypothesis: no relationship between years of education and income ρ education*income = 0 Research hypothesis: relationship between years of education and income r education*income 0

7 r xy = n ΣXY - ΣX ΣY [n ΣX 2 – (ΣX) 2 ][n ΣY 2 - (ΣY) 2 ] r xy = correlation coefficient between x & y n = size of sample X = score on X variable Y = score on Y variable

8 6. Determine whether the statistic exceeds the critical value.95 >.81 &.92 So it does exceed the critical value 7. If over the critical value, reject the null & conclude that there is a relationship between years of education and income

9 In results There was a significant positive correlation between years of education and income, such that income increased as years of education increased, r(4) =.95, p <.05 (can say p <.01).

10 Even though you may suspect theres a causal relationship you can only make causal statements if: X definitely preceded Y X was manipulated so that it was the only probable factor that could cause changes in Y When talking correlations, you can use relationship, relate, associated

11 Years of Education Income (in 1,000s)

12 yrsedincome yrsedPearson Correlation 1.95(***) Sig. (2-tailed).000 N 6 6 incomePearson Correlation.95(***)1 Sig. (2-tailed).000 N 66

13 .80 to 1.0Very strong.60 to.80Strong.40 to.60Moderate.20 to.40Weak.00 to.20Weak/ None

14 Coefficient of Determination Percentage of variance in one variable that is accounted for by variance in the other Square the correlation coefficient If r =.70 r 2 =.49 49% of variance is shared (or variance in one is explained by variance in other)

15 Cases/ people will have different scores on measures Correlations reflect the extent to which peoples scores tend to move together Variables are correlated if they share variability So coefficient of determination estimates how much of the differences among people on one measure is associated with differences among people on the other measure


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