SPSS 2: item analysis and cronbach's alpha

SPSS 2: item analysis and cronbach's alpha

1. yields a table of correlations between each item and the overall total for the questionnaire. (item total correlations)

   • cronbacks alpha - table of values showing what each alpha would be if deleted.

2. negative item-total correlations = either recode to see if it makes an improvement; if still not then delete.

   • small item-total correlations and poor reliablility = if alpha is low (below 0.7) you need to delete the items with the smallest contribution to see if it affects overall score
     • if overall score is 0.7 keep items which are weak
   • only re-code an item once

3. SPSS step one = recode intentionally negative items. Transform > recode into a different variable

   • SPSS step two: initial Cronbach's alpha. Analyze > Scale> Reliability Analysis. Statistics > Scale if item deleted. continue
     • SPSS step three: further recoding. WORK WITH ONE ITEM AT A TIME. 1st - recode the item with the largest negative item-correlation. repeat until 0.7.
       • SPSS step four: DELETING ITEMS. start with highest alpha if deleted. continue until 0.7
         • ties? if same cronbach's when deleted look at the question wording or lowest item-total correlation.

4. Correlation

   • SPSS step one:create a scatter plot: graphs> chart builder.
     • SPSS step two- choose the chart. e.g. scatter, select and drag to preview area.
       • from 'Variables' panel drag and drop each variable onto the axis
   • correlational analysis. Analyze> Correlate> Bivariate
     • Bivariate correlations. select dependent variavles and move into variables panel
       • choose which correlation test to run, Pearsons or Spearmans. choose two tailed or one tailed.
         • writing up = r=.733, N=25, p<.001. The relationshup accounts for 54%
           • PARTIAL CORRELATION: Analyze> Correlate> Partial
             • 1st = dependent variavle into variables. then choose which variable to control.
   • multiple correlation = adding all the variables into the dialog box at once.

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