[Rasch] negative alpha

Parisa Daftari Fard pdaftaryfard at yahoo.com
Sat Dec 6 13:35:40 EST 2014


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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