Data for the period 2015-04 to 2015-09 was analyzed in order to examine the possible relationship between teat wash failure and the result on milk yield for dairy cows. Data provided by Sveriges Lantrbruksuniversitet over 49 093 specific milking events were used. Two linear mixed-effects models and one basic OLS-model were estimated. In order to perform the analysis a lot of data manipulation also had to be performed. The data analysis was divided into to two parts. First the variable of interest (teatwash) was examined by constructing two versions of the different models; an unrestricted- and a restricted version were teatwash had been excluded. Because of the large sample and linear mixed-effect models an out-of-sample forecasting method was used as the primary evaluation criteria. The prediction errors were evaluated on the basis of root mean squared error (RMSE) and mean squared error (MSE). The difference between the unrestricted- and restricted models was very small and no indication of a relationship between teat wash failure and milk yield was found. The second part involved the comparison of prediction errors between the two mixed-effect models and the OLS-model. Surprisingly, the basic OLS-model resulted in the lowest prediction error although obvious breach of assumptions.