Questions to be addressed

Question1

Lactation curves were plotted in the paper for each of the three sets of data: baseline survey questionnaire, longitudinal survey and post-survey questionnaire.

Each set of data was analysed using Wood's model for a lactation curve:

y_n = an^be^-cn,

where y_n is the milk yield on day n and a, b and c are constants.

However, the structures of the data sets collected at the different times are different: one longitudinal and two cross-sectional. Wood's curve is strictly suitable only for repeated milk yields from groups of individual cows and is unable to handle the large variation in milk yield from cow to cow that cannot be distinguished within a set of single observations.

Therefore the first question that we shall address is:

• Are the curves produced in Figure 2 of Romney et al.'s paper appropriate representations of the patterns in the three years?