Using SPSS to Compare Means

UDP 520 Lab 4Lin Lin

November 6th , 2007

Outline

• Dataset

• Independent samples

• Paired samples

Dataset—Weight Lost Training Program (WLTP)

• 1000 adults aged 18+ (including both males and females) were recruited to study the effectiveness of Weight Lost Training Program.

• Variables– Sex– BMI_1(before WLTP)– BMI_2(after WLTP)

• Download dataset fromhttp://courses.washington.edu/urbdp520/UDP520/WLTP_new.sav

2 2

( )BMI=703

( )

Weight lb

height in

Questions

• Question one: – Is BMI significantly different between

males and females before WLTP?

• Question two: – Overall (for both males and females), is

WLTP effective?

Question One

• Step one: Making assumptions and meeting test requirements – Random sampling– Level of measurement is interval-ratio– Sampling distribution is normal

• Step two: stating the null hypothesis

• Step three: selecting the sampling distribution and establishing the critical region – Sampling distribution = Z distribution – Alpha=0.05, two-tailed– Z(critical)= 1.96

0

a

H :

H :

males females

males females

u u

u u

Question One (cont.) Step four: computing the test statistic in SPSS

Question One (cont.)

• Step five: making a decision and interpreting the results of the test

Independent Samples Test

2.313 .129 -.989 998 .323 -.07871 .07960 -.23492 .07749

-.990 996.636 .323 -.07871 .07953 -.23477 .07735

Equal variancesassumed

Equal variancesnot assumed

BMI_1F Sig.

Levene's Test forEquality of Variances

t df Sig. (2-tailed)Mean

DifferenceStd. ErrorDifference Lower Upper

95% ConfidenceInterval of the

Difference

t-test for Equality of Means

resultor Z (obtained)

Question Two

• Step one: Making assumptions and meeting test requirements – Random sampling– Level of measurement is interval-ratio– Sampling distribution is normal

• Step two: stating the null hypothesis

• Step three: selecting the sampling distribution and establishing the critical region – Sampling distribution = Z distribution – Alpha=0.05, two-tailed– Z(critical)= 1.96

0

a

H :

H :

before after

before after

u u

u u

Question Two (cont.) Step four: computing the test statistic in SPSS

Question Two (cont.)

• Step five: making a decision and interpreting the results of the test

Paired Samples Correlations

1000 .657 .000BMI_1 & BMI_2Pair 1N Correlation Sig.

Paired Samples Test

.42943 1.47901 .04677 .33765 .52121 9.182 999 .000BMI_1 - BMI_2Pair 1Mean Std. Deviation

Std. ErrorMean Lower Upper

95% ConfidenceInterval of the

Difference

Paired Differences

t df Sig. (2-tailed)

resultor Z (obtained)

Exercises

• Is BMI significantly different between males and females after WLTP?

• Is WLTP effective for males? – Hint (do selection in “Data”— “Select

cases”— “if condition is satisfied”— “if”— “sex=0”)

• Is WLTP effective for females?

Selecting cases in SPSS