Measurement error in direct observations: a comparison of common recording methods

Videotapes of three brief duration, three medium duration and three long duration types of stereotyped behaviour (of eight severely retarded children) were analysed to provide a criterion record of the true percentage duration of the behaviour. The criterion record was compared with the records produced by four time-sampling methods: a whole-interval method, two partial-interval methods and a momentary time-sampling method. As predicted, the whole-interval method grossly underestimated and the partial-interval methods grossly overestimated the true percentage duration of the behaviour, except when the duration of individual responses was much longer than the observation interval. Momentary time-sampling was not an errorless method but was consistently superior to the other methods.The implications of these findings for the detection of treatment effects by direct observations are discussed.