Study questions
- Write in your own way a brief report of the data
analysis on milk offtake and the findings of the survey on consumption and
marketing for an agricultural extension worker who may not be familiar with such
statistical terminology as regression lines or standard errors. Discuss the
adaptations that you have made to the report given in the case study.
- Discuss the suitability of the cross-sectional approach
used here for estimating milk offtake in relation to the objectives outlined
earlier. Are there other types of study designs that you might recommend as being
more suitable, either for this study or for any follow-up? Choose one
alternative approach, describe, in general terms, how the study would be
organised and discuss any advantages or disadvantages from that carried out
here.
- Exclude the two high milk offtakes in early lactation.
Rerun the GenStat regression analysis and compare with the previous output.
Which results do you think you should present? Discuss under what circumstances
it is permissible to exclude outliers.
- Exclude the two milk offtakes with the highest
leverages and rerun GenStat. What do you deduce from this output? Should these
points be omitted?
- The general relationship between milk offtake and stage
of lactation in cows is known to be curvilinear, increasing from calving to a
peak value around 4 − 6 weeks and then decreasing. Include a quadratic term
for total milk offtake in the model and rerun GenStat. Does the analysis suggest
that a quadratic term should be included? Do you think that the type of data
collected provides the best way for determining the shape of the lactation
curve? If not, how would you design a study to achieve this aim?
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