This paper deals with the reliability of composite measurement consisting of true-false items obeying the Rasch model. A definition of reliability in the Rasch model is proposed and the connection to the classical definition of reliability is shown. As a modification of the classical estimator \textit{Cronbach's alpha}, a new estimator \textit{logistic alpha} is proposed. Finally, the properties of the new estimator are studied via simulations in the Rasch model.