Analysis of Repeated Measures Data of Iraqi Awassi Lambs Using Mixed Model

Abstract

In this study, repeated records of body-weight of Awassi lambs were considered for analysis. Records included up to five ‘repeated records’ of body-weight per lamb, measured between birth weight and 4th month of age, were used in the analysis. Most statistical approaches in such data are based on analysis of variance (ANOVA). However, the assumption that datum are independent is usually violated since several measures are performed on the same subject. As a result, standard regression and ANOVA may produce invalid results of repeated measures data because they require mathematical assumptions that were inconsistent with repeated data. The newest approach to analyzing of the repeated measurements is a mixed-model analysis. Advocates of this approach claimed that it provides the “best” approach to the analysis of repeated measurements. Therefore, the objective of the study was to investigate the effect of flock on growth performance of Awassi lambs using the mixed model. Three models was used: the first model consist of the effect of flock, time and flock by time interaction, the second model includes the same factors besides the quadratic effect of time, and the third model includes all factors in second model besides the time by time by flock interaction. Results revealed that the third model was better than other models and the effect of all factors on body weight of lambs was significant (P< 0.05) except the effect of flock, which was non-significant.