====== Documentation for online alpha calculation ======

[[https://websem.psychstat.org/apps/alpha/|Go back to the interface]]

===== Interface =====

The online alpha has an interface like below

{{:research:webinterface.jpg}}

===== Options =====
==== alpha or omega ====
Once can choose to calculate Cronbach's alpha or McDonald's omega here.

==== Data File ====
The data file has to be in free format, separated by space. Each column represents an item. Variable names can be specified in the first line of the data file. An example file can be downloaded at {{:research:spatial.txt}} with the first several rows of data given below.

<code>
visual cubes paper lozenge paperrev flagssub
23 19 13 4 13 25
33 22 12 17 20 37
34 24 14 22 19 40
29 23 12 9 11 44
16 25 11 10 10 28
</code>

=== Drop cases ===

The software allows a user to drop cases, e.g., remove cases that are identified as outlying observations. The cases to drop are specified using their case numbers separated by space. For example, 
<code>
1 5  7
</code>
means the cases 1 5 and 7 are dropped. 

==== Missing data ====
Missing data can be dealt with using the robust algorithm (ML) or listwise deletion (removing cases with missing data).

==== Downweight rate =====
The tuning parameter to downweight data. The default is 0.1.

==== Standard error =====
Whether to estimate standard error for alpha. It can be slow for a large set of items.

==== Plot =====
No plot, Diagnostic plot, Weight plot, Profile plot, or all three plots.

Profile can specify how many cases to be highlighted in the weight and profile plots. Default is 5.

===== Running time =====
The program can take a significant time to run. If it is this case, refresh your web browser to display results.

===== Sample output =====
The output typically includes the alpha estimates, the plots, and a link to pdf plot. A screenshot is given in the figure below.

{{:research:weboutput.png}}