[Rasch] negative alpha

Tom Conner connert at msu.edu
Sat Dec 6 13:43:51 EST 2014


Thinking substantively, it could be that the test was too easy for the 
population, that there was cheating, or some other aspect of the 
administration or population.

On 12/5/14, 9:35 PM, Parisa Daftari Fard wrote:
> Thank you John and Mike
>
> Mike
>
> What actually happened that SEM is more than STD. Does it mean that 
> some students were careless to answer and did it by chance. How can 
> this be corrected?
>
> Parisa
>
>
> On Saturday, December 6, 2014 1:01 AM, Mike Linacre 
> <mike at winsteps.com> wrote:
>
>
> Parisa:
>
> In your report, the Std. Dev. (observed standard deviation) is less 
> than the SEM (standard error of measurement). The dispersion of the 
> examinees is less than what is expected by chance.
>
> Alpha (Reliability) = (Std. Dev.**2 - SEM**2) / Std. Dev.**2
>
> Mike L.
>
> On 12/6/2014 6:54 AM, Parisa Daftari Fard wrote:
>> Dear Friends,
>>
>> I have run itemman on a set of data and come up with the following 
>> result. Why alpha turned to be negative? I appreciate your incisive 
>> comments on the result.
>>
>> N of Items           8
>> N of Examinees      22
>> Mean             2.864
>> Variance         1.027
>> Std. Dev.        1.013
>> Skew             1.061
>> Kurtosis         0.006
>> Minimum          2.000
>> Maximum          5.000
>> Median           3.000
>> Alpha           -0.754
>> SEM              1.342
>> Mean P           0.358
>> Mean Item-Tot.   0.263
>> Mean Biserial    0.337
>> Max Score (Low)      2
>> N (Low Group)       10
>> Min Score (High)     3
>> N (High Group)      12
>>
>
>
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>
>
>
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-- 
Tom Conner
Professor Emeritus
Department of Sociology
Michigan State University

"I contemplate with sovereign reverence that act of the whole American people which declared that their legislature should 'make no law respecting an establishment of religion, or prohibiting the free exercise thereof,' thus building a wall of separation between Church & State." Thomas Jefferson

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