Cronbach's Alpha: Measuring Reliability
Cronbach's alpha reliability coefficient (or Cronbach's alpha for short) ranges from .00 (very unreliable) to 1.00 (perfectly reliable). Cronbach's alpha summarizes the correlation ('relatedness') of every opinion in the cluster with every other opinion in the cluster. The answers to all opinions in a cluster can be combined to form an index. For example, the average or mean of all items that make up a cluster can be used as an index to measure that belief system. All social scientists would agree that a Cronbach's reliability coefficient for an index of 1.00 would be ideal. However, few indices are that reliable. For an example of a perfectly reliable index, click here.
In general, Cronbach alphas in the .90 to .99 range are considered great. These indices or scales are very reliable. Agreement with any single opinion in the index implies agreement with the other items in the index. By the same token, disagreement with any single opinion implies disagreement with the other opinions. For an example of an index with a great Cronbach's alpha (.92), click here.
Cronbach's alphas in the .80 to .89 range are considered good. Agreement with one opinion in the index generally indicates agreement with the others.
Indices in the .80-89 range are not as consistent as with indices in the .90-.99 range, but these alphas are still considered good. All indices reported for the Death Penalty Study have Cronbach's alphas of .80 or higher.
Cronbach's alphas in the .70-.79 range are okay for some purposes. In general, researchers with indices in the .70-.79 range would work to devise an index with greater reliability.
Cronbach's alphas below .70 are not reliable and, in general, should be avoided.