Showing posts with label Psychological test scores. Show all posts
Showing posts with label Psychological test scores. Show all posts

Wednesday, August 28, 2019

Why Counselor's Tests Are Not Reliable





The reason counselor's tests are not reliable is that reliability is a property of scores not tests. This isn't a matter of semantics. Think about it this way.

Give all the students in one school an achievement test. The test items don't change so they appear stable, consistent, and reliable. However, when publishers report reliability values, they calculate the reliability statistics based on scores. Scores vary from one administration to another. If you ever took a test twice and got a different score, you know what I mean. Individuals change from day to day. And we change from year to year. Also, even a representative sample of students for a nation can be different each year.

Everytime we calculate a reliability statistic, the statistic is slightly different.

Reliability values vary with the sample.

Reliability values also vary with the method used for calculation. You can get high reliability values using coefficient alpha with scores from a one-time administration. This method is common in research articles. But you will see different values from the same research team in different samples in the same article.


If we use a split-half method, which usually calculates reliability based on a correlation between two halves of one test, then we can get a reliability value based on one administration. But that's only half a test! Researchers use the Spearman-Brown formula to correct for the shortened half-test problem- but that's just an estimate of what the full test could be.


There's also a test-retest reliability method. Give a test one time, wait awhile- maybe a week or several weeks, then retest. That gives you an estimate of stability. But if you have a good memory, you can score higher on the second test on some tests like intelligence and achievement.


By now you get the point. Any one test can be associated with a lot of reliability values. The problem is not necessarily with counselor tests. The problem can be misunderstanding that tests do not have one reliability value. As with many things in science, there are many variables to consider when answering a question.

Reputable test publishers include reliability values in their test manuals. Counselors, Psychologists, and other test users ought to know about test score reliability.

Learn more assessment and statistical concepts in

Applied Statistics: Concepts for Counselors

AMAZON BOOKS




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Wednesday, October 25, 2017

How to Compare Test Scores

















When counselors and psychologists report test scores, they often report one of the scores found in the table below. When several tests are used, it is helpful to know how the scores compare from one test to another.

A good place to begin is to locate the average score-- that's the row where z = 0. Then look at the broad middle range between z = -1 and z = 1. About 68% of people score between z = -1 and z = 1.

Intelligence Tests use Standard Scores abbreviated as SS. These scores take the place of the old IQ score. An average IQ is 100 -- about 68% of people score between 85 and 115.


Here's a table from Appendix B of Applied Statistics: Concepts for Counselors



Each row contains the equivalent score on a different scoring system. For example, a z-score of 1 equals a T score of 60, and a standard score of 115. The score is at the 84th percentile.


z
T
Standard
Percentile
Rank
3
80
145
99.9
2.5
75
138
99.4
2
70
130
97.7
1.5
65
123
93.3
1
60
115
84.1
.5
55
108
69.2
0
50
100
50.0
-.5
45
93
30.8
-1
40
85
15.9
-1.5
35
78
6.7
-2
30
70
2.3
-2.5
25
63
0.6
-3
20
55
0.1




Table Notes

z-scores, M = 0, SD = 1
T-scores, M = 50, SD = 10
SS-Standard Scores, M = 100, SD = 15
PR- Percentile Rank


 Find more practical concepts in Applied Statistics: Concepts for Counselors

An inexpensive reference book for students and counselors















Learn more at the book's website  counselorstatistics

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