[Psychometric properties of a scale: internal consistency].

Internal consistency reliability is the degree of correlation between a scale's items. Internal consistency is calculated by Kuder-Richardson's formula 20 for dichotomous choices and Cronbach's alpha for polytomous items. 0.70 to 0.90 internal consistency is acceptable. 5-25 participants are needed for each item when computing the internal consistency of a twenty-item scale. Internal consistency varies according to population and then it is necessary to report it always that scale is used.