lab 04 2007

8/13/2019 Lab 04 2007

1/16

Lab 4: Alpha and Standard Errorof Measurement

8/13/2019 Lab 04 2007

2/16

8/13/2019 Lab 04 2007

3/16

8/13/2019 Lab 04 2007

4/16

Calculating Alpha of the

Extraversion Scale for the Time 2 IPIP

SPSS: AnalyzeScaleReliability Analysis

Put the following items in the items box: e1_t2, e2_r_t2, e3_t2, e4_r_t2, e5_t2, e6_r_t2, e7_t2, e8_r_t2,

e9_t2, & e10_r_t2

Make sure you use the reverse scored items!

Click on Statistics

Inter-Item: Correlations Summaries: Correlations

Then hit Continue

Then click OK

8/13/2019 Lab 04 2007

5/16

Questions to Answer from Output

1. What was the average inter-item

correlation? 2. How many items are on the scale?

3. What was the standardized alpha?

8/13/2019 Lab 04 2007

6/16

Calculating Alphas for the

other four scales (A, C, N, and O)

Agreeableness: a1_r_t2, a2_t2, a3_r_t2, a4_t2,a5_r_t2, a6_t2, a7_r_t2, a8_t2, a9_t2, & a10_t2

Conscientiousness: c1_t2, c2_r_t2, c3_t2, c4_r_t2,c5_t2, c6_r_t2, c7_t2, c8_r_t2, c9_t2, & c10_t2

Neuroticism: n1_t2, n2_r_t2, n3_t2, n4_r_t2, n5_t2,

n6_t2, n7_t2, n8_t2, n9_t2, & n10_t2 Openness: o1_t2, o2_r_t2, o3_t2, o4_r_t2, o5_t2,

o6_r_t2, o7_t2, o8_t2, o9_t2, & o10_t2

8/13/2019 Lab 04 2007

7/16

What happens if you forget to reverse

score the items?

Calculate alpha for the Extraversion scale

again, but now use the following items: e1_t2, e2_t2, e3_t2, e4_t2, e5_t2, e6_t2, e7_t2,

e8_t2, e9_t2, & e10_t2

What is the new alpha? How can you tell that you did something

wrong?

8/13/2019 Lab 04 2007

8/16

What happens if you calculate alpha of

two scales at the same time?

Calculate alpha for the following eight items:

e1_t2, e2_r_t2, e3_t2, e5_t2 (Extraversion items)

c1_t2, c3_t2, c5_t2, c7_t2 (Conscientiousness items)

Can you interpret your result? What might clue you

into your mistake?

Remember: Alpha does not indicate

unidimensionality!

8/13/2019 Lab 04 2007

9/16

Standard Error of Measurement

Standard deviation of the error around anyindividuals true score

If a student were to take the same test repeatedly,with no change in his level of knowledge andpreparation, it is possible that some of the resultingscores would be slightly higher or slightly lower than

the score that precisely reflects the students actuallevel of knowledge and ability.

The difference between a students actual score andhis highest or lowest hypothetical score is known asthe standarderrorof measurement.

8/13/2019 Lab 04 2007

10/16


False Negative

A small percentage of students may score lowerthan anticipated on a test

False Positive

A small percentage of students may score higherthan otherwise would have been expected

8/13/2019 Lab 04 2007

11/16


Using a reliability coefficient and the tests

standard deviation, we can calculate this value:

Where: sx= the standard deviation for the test

rxx= the reliability coefficient (alpha) for the test

1x xxSEM s r

8/13/2019 Lab 04 2007

12/16


Standard Error of Measurement for Agreeableness Scale

To obtain rxx: Repeat steps from earlier in Lab

AnalyzeScaleReliability Analysis

Enter all Agreeableness items for Time 2

Click on Statistics

Inter-Item: Correlations

Summaries: Correlations

Then hit Continue

Then click OK

1x xxSEM s r

8/13/2019 Lab 04 2007

13/16


To obtain standard deviation:

AnalyzeDescriptiveDescriptives

Enter a_t2 (non-standardized)

Check standard deviation in options

Click Ok

Hence:

SEM= .55 1-.86 = .20

8/13/2019 Lab 04 2007

14/16

SEM Confidence Interval

96% Confidence: Z = 2.0

68% Confidence: Z = 1.0

96% Band = Score (2 *SEM)

CI = 4

(2 * .20) = 3.6 - 4.4

68% Band = Score (1 *SEM) CI = 4 (.20) = 3.84.2

SEMZScoreObservedCI Confidence

8/13/2019 Lab 04 2007

15/16

Homework Questions 1 & 2

1. Report the standardized alphas for all fiveIPIP scales at Time 2. Also report theaverage inter-item correlation and number ofitems. (2 point)

2. Imagine that the average inter-item

correlation for a scale is .40. Compute thestandardized alphas for tests with thefollowing numbers of items: 5, 10, 15, 20, 25.(3 points)

8/13/2019 Lab 04 2007

16/16

Homework Questions 3 & 4

3. Report the SEM for all 5 IPIP scales at Time

2 with interpretation. (3 points)

4. Report the confidence interval for the mean

conscientiousness at time 2 at the 96% CI

with interpretation. (2pts.)

lab 04 2007

Documents