I have an exam on monday and need to know how to calculate Cronbach's alpha.

I know that you have to use the spearman brown formula when you shorten your scale. I can't quite figure out what to do when the scale gets longer.

Here is the example question I struggle with:

Imagine Cronbach's alpha for research A was 0.7. Which value for Cronbach's alpha can we expect when we extend the scale with 10 parallel items (the final scale has 15 items).

The expected value will be:

a) 0.44

b) 0.82

c) 0.88

d) 0.97

Does anybody know the formula I have to use in this case and how to calculate this ?

Thanks for helping!!