[Rasch] negative alpha

Parisa Daftari Fard pdaftaryfard at yahoo.com
Sun Dec 7 00:21:01 EST 2014


Thank you Prof Conner,
Is it possible to find out cheating cases through statistics?
BestParisa 

     On Saturday, December 6, 2014 6:14 AM, Tom Conner <connert at msu.edu> wrote:
   

  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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 -- 
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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