You
will find the data in a file named PSY621.XLS. If you
right click, and choose the "Save Target As" option, you can save it
to your local machine.
These
data are from a group of nearly 500 subjects who filled out a 122-item survey
which is available here. This survey is actually four questionnaires:
1. Perceptual Aberration (PA): taps body
image aberrations and distortions.
2. Magical Ideation (MI): taps
"magical thinking" and other beliefs that would not be endorsed by
many people in this culture.
3. Impulsive Nonconformity (NC): taps
impulsivity with special disregard for social norms.
4. Infrequency (IF): a seven item scale
designed to identify careless responding and malingerers.
If
the item was endorsed "true", the code is 1; if "false",
the code is 0. Missing data (omitted
items) are indicated by spaces. You will
need to decide how to handle missing data.
Most
items are keyed "True"; i.e., endorsing true gives the person one
point towards the total score, endorsing false does not. Some items, however, are keyed
"False"; endorsing false gives the person one point towards the total
score, endorsing true does not. The
following items are keyed false and therefore must be recoded: 4, 6, 14, 26, 27, 28, 35, 38, 40, 46, 48, 49,
52, 53, 54, 56, 58, 59, 76, 84, 101, 103, 104,
106, 112.
The
following items are on each scale (also indicated on the attached
questionnaire):
Perceptual
Aberration: 2, 3, 5, 7, 9, 13, 16, 18, 22, 23, 26, 29, 31, 34, 37, 39, 41, 44,
47, 48, 52, 55, 59, 60, 66, 70, 71, 77, 94, 96, 98, 102, 108, 109, 118.
Magical
Ideation: 06, 08, 10, 11, 25, 27, 36, 46, 50, 51, 54, 56, 61, 65, 67, 73, 75,
76, 78, 79, 81, 83, 88, 92, 104, 116, 117, 119, 120, 122.
Impulsive
Nonconformity: 01, 04, 12, 14, 15, 17, 19, 20, 24, 28, 30, 32, 35, 38, 40, 42,
43, 45, 49, 53, 57, 58, 63, 68, 69, 72, 74, 80, 84, 85, 86, 87, 89, 90, 91, 93,
95, 99, 100, 101, 103, 105, 106, 107, 110, 111, 112, 113, 114, 115, 121.
Infrequency:
21, 33, 62, 64, 82, 97.
Before
starting the assignment, also determine whether you will exclude individuals
based on aberrant scores on the Infrequency scale, and justify your decision
and approach to exclusion.
Assignment
First
you should decide what subjects to include and exclude and justify your
decision. Then on the dataset of included subjects, address the following
items. For each item below, please include all relevant printouts and,
when performing calculations by hand, please include the formula you choose to
use and show your calculations.
1. Compute the total
score for each of the four scales, then obtain and print out the
intercorrelation matrix for these four scales (a 4x4 matrix of
correlations). This is a single-method multitrait matrix.
What does this tell you about the homogeneity or heterogeneity the
constructs tapped by these scales?
2. What are the
standard errors of these correlations (assuming that r is an
unbiased estimate of ρ)?
3. Compute and print
out the item statistics (p and rit). Identify poor items on the printout, and
indicate in general what you decided would constitute a bad item. Articulate your rationale for your decision
about the criterion/criteria for a bad item.
4. Compute and print
out the internal consistency (rtt)
of each of the four scales by whatever method you deem appropriate (justify
your deeming).
5. Fred scored 22 on
the Perceptual Aberration Scale; Clyde scored 13. Is the difference between their scores
statistically significant at the .05 level?
6. What do you
estimate the reliability of the difference (rDD)
between the Perceptual Aberration and Magical Ideation Scales to be?
7. By hand, recompute
the correlation matrix from question 1, correcting
for attenuation by the unreliability of each scale. Now, what would you say about the homogeneity
or heterogeneity the constructs tapped by these four scales?
8. By hand, compute
the estimated reliability of each scale if you doubled the length of the scale
by adding items comparable to these existing items in terms of their
psychometric properties and content area.
9. Exclude the items
you identified as poor on each of the scales (excluding the infrequency
scale). Then recompute the internal
consistency estimates (rtt)
of each of these three scales by whatever method you already deemed
appropriate.
10. Now assume that
you added new items that are of comparable quality to these retained
items. Assume you add as many items to
each scale as you excluded in # 9 above.
By hand, what would you estimate the internal consistency estimates (rtt) of each of these three
new-and-improved scales to be?