In another one of your studies you have resources to include only a shortened version of the collective efficacy scale in your survey. You hope to cut the scale approximately in half.

What would be the estimated coefficient alpha if you used only 10 of the 20 items (show your calculations)?

Sampson RJ, Raudenbush SW, Earls F. Neighborhoods and Violent Crime. A Multilevel Study of Collective Efficacy, Science 277(August 15, 1997):1 -7.