The reliability and stability of various individual difference and tolerance to shiftwork measures

The aim of the study was to evaluate the reliability and temporal stability of various individual difference and tolerance to shiftwork measures which were used in an extensive study on relationships between shiftworkers' features and tolerance to shiftwork. The study was conducted in a Croatian oil refinery where 604 male shift workers, working in a 2:2:3 continuous three-shift system were examined by means of various questionnaires. A subsample of 61 shift workers were re-examined using the same questionnaires and the same administration procedure nine years later. The questionnaires were: Jenkins Activity Survey, Eysenck Personality Inventory, Momingness-Eveningness Questionnaire, Behavioural Arousal Questionnaire, Orcadian Type Questionnaire and Inventory, Way of Life Questionnaire, Health Information Questionnaire, Sleep Quality Scale. The questionnaires were scored for 14 individual difference measures and eight tolerance to shiftwork measures. In addition to these scores sleep durations on night, morning, afternoon shifts, and on days off were used as tolerance to shiftwork measures. The reliability of each measure was determined by means of Cronbach alpha coefficients computed on a total of 604 shiftworkers and temporal stability by means of correlation coefficients between the scores obtained on two occasions on the subsample of 61 shiftworkers. The internal consistency coefficients for most of the measures ranged between moderate to high, and were considered to be satisfactory. The correlations between the scores obtained twice over a period of nine years proved both the individual difference and tolerance to shiftwork measures to be temporally stable. The only measure that had insignificant correlation was the sleep duration on days off.