Correlation

Median HHI and Percent of Households Eligible for Food Stamps

Percent of Households Eligible for Food Stamps and Percent Unemployed

A few words about Scatterplots A Scatterplot displays the direction and strength of the association between two sets of quantitative data The direction refers to whether there is a positive, negative, or no association between the data The association is strong if the dots come close to being in a straight line The association is weak if the dots don’t come close to being in a straight line

A number called the correlation measures both the direction and strength of the linear relationship between two related sets of quantitative variables.

Properties of Correlation The correlation is written as r Correlation requires that both variables be quantitative A positive value for r means there is a positive relationship between the variables A negative value for r means there is a negative relationship between the variables

The value of the correlation is always between - 1 and + 1. A strong relationship means the correlation is close to either - 1 or + 1 There can be a strong positive relationship (r is close to 1), or a strong negative relationship (r is close to - 1) A weak relationship means the correlation is closer to 0 than to either - 1 or + 1 The value of r is affected by outliers

Guess the Correlations:

Excel Formula for Correlation: =CORREL(array1,array2) AB =CORREL(array1,array2) =CORREL(a1:a6,array2) =CORREL(a1:a6,b1:b6)

Percent of HH Eligible for Food Stamps New AIDS Cases per 100,000 (2000) Percent of 35 – 44 y/o with Bachelor ’ s Degree Percent Unemployed Percent Unemployed and Not in Labor Force Spending per Student (Grades 1 – 8) Median HHI (1999) Percent of HH Eligible for Food Stamps New AIDS Cases per 100,000 (2000) Percent of 35 – 44 y/o with Bachelor ’ s Degree Percent Unemployed Spending per Student (Grds 1 – 8)