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Exploration & description The number of lambs characterised by age of dam also reveals a frequency imbalance. The oldest Red Maasai ewes were aged 6 years whereas one Dorper ewe was as old as 10 years. From the numbers of lambs for each age category it can be seen that dams between the ages of 2 and 6 years were most common. Extreme age classes of 1, 9 and 10 years had only one lamb each. Since age of dam is a factor to be considered in the analysis of weaning weight of lambs, it would not be sensible to keep these classes separate. One can either omit these three records or pool them with existing ones. We have chosen to put age 1 year and 2 years together to form one class (2 years and below) and to put ages 9 and 10 years together into the age 8 year category to form an '8 years and above' class. The column DAMAGE7 in CS3Data has been created using the GenStat command Spread → Factor → Change levels… to put the extreme values into the neighbouring categories.
When fitting a classification factor in a statistical model it is always important to check that there are reasonable numbers of observations within each category level; attempts to fit parameter terms to sparse data often leads to spurious estimates. |
