Scale reliability (Cronbach's alpha)

Several questions are meant to measure one thing together, for example enjoyment of learning. Cronbach's alpha shows the internal consistency: how strongly the answers to these questions are related. It usually lies between 0 and 1. Common rule of thumb: from .70 acceptable, from .80 good, from .90 excellent.

Example: Four questions about enjoyment of learning with α = .77: the internal consistency is acceptable.

Caution: Alpha rises just by adding more questions. It does not show whether the questions measure one common trait (unidimensionality) - a factor analysis does that - nor whether they measure the right thing. A question with a negative item-total correlation is often worded the other way round and needs to be reverse-coded.