Ch 8 Reliability of Instrument
Definition

Measures the extent to which the instrument yields
consistent results when the variable (s) and / or
characteristics being measured haven’t changed.
Types of Instrument Reliability
1.
2.
3.
Stability Reliability
Alternate Form Reliability
Internal Consistency Reliability
Stability Reliability
AKA test-retest reliability
 Consistency of measurement over time
 Classification consistency

Alternate Form Reliability
The consistency of results between different forms of
the same test
 Score Correlation
 Difficulty

What will happen if you combine stability
reliability and alternate form reliability?
Determining Test Difficulty

Field testing

Embedding
Internal Consistency Reliability

The degree to which all items on the test measure
assessment domain consistently
Spearman Brown
2x reliability of ½ test
 Reliability
=
1+reliability on ½ test
K-R20 and K-R21
 K-R21=
K
 M (K  M ) 

K  1 
Ks 2
Used with dichotomous items (eg: yes/no answers)
 K= number of items on test
 M= mean of the set of test scores
 S= standard deviation of the set of scores

Standard Error of Measurement

S e = S x 1  rxx
An index of the consistency of an individual’s test
performance
 An estimate of how an individual’s score would vary if
they took the test again
 Confidence interval or band

Ways to Insure Reliability
Establish criteria ahead of time to base any decisions on
 Be consistent in the way you administer the instrument
 Anyone assisting in the research should be well trained
