Data management/Exploration & description

We shall first check the summary statistics for lactation length via Stats → Summary Statistics → Summarize Contents of Variates…. The output shows a range in lactation lengths from 152 to 275 days and a mean of 221.8 days.

Dividing mean lactation length (days) by 14 we get a mean of 15.8 fortnights. Thus, if we make an arbitrary decision and use records up to M15 - the measurement at the 15th period of lactation (see CS2Data1) we can assume a truncated lactation of
15 x 14 = 210 days. 

Summary statistics for LACLEN

Number of observations

 = 387

Number of missing values

 = 0

Mean

 = 221.8

Median

 = 226.0

Minimum

 = 152.0

Maximum

 = 275.0

Lower quartile

 = 204.0

Upper quartile

 = 243.0

The above formula assumes that the average number of days to the first recording after calving, namely M1, will have been anywhere between one and 14 days (average 7 days) after calving, and that the drying off date would have occurred anywhere between one and 14 days after M15 (again average 7 days). Thus each recording can be assumed to be on average at the mid-point of a 14-day period.

Having defined a suitable subset of the data it is much safer to continue to carry out calculations in a worksheet different from one that contains the original raw data. Therefore, we copy the whole data set by File → Save as… into CS2Data2, close CS2Data1, open CS2Data2,  repeat the calculations to omit the first parity cows, and then, by using Spread → Delete → Restricted Rows we can keep just the data for 387 cows (having already omitted Cow 190) in at least their second lactation.