THE SIZE AND COST OF THE ESTIMATION ERROR WHEN USING ANALYSIS OF A SAMPLE OF A DAIRY-HERD TO ASSESS THE WHOLE HERD PERFORMANCE

Whilst computerised herd health schemes have been available for almost 20 years in the UK and they have been shown to increase profitability by pound 45-65 per cow per year their commercial uptake has been disa ppointing. Previous surveys have shown that uptake is limited due to p oor motivation amongst farmers who fail to see either the need or the benefits. In order to demonstrate the potential benefits to a farmer a certain amount of data analysis is required but manual data analysis is limited by the size of commercial herds and the amount of data invo lved, The analysis of a sample of the herd would be possible but the a ccuracy of the results is not known. Five hundred repeat samples of 10 , 20 and 40 cows were taken from six large commercial herds (average N = 218, range 175-245), and the 95% confidence interval for fertility and disease parameters calculated. The estimation error (with 95% conf idence limits) about the mean was expressed in financial terms using s tandard costs for performance shortfalls, For samples of 20 cows the e stimation error averaged pound 39.20 for ten parameters over the six h erds. For a sample size of 40 cows, the average was pound 26.39. The r esults indicate that sampling 40 cows from large commercial herds (abo ut 200 cows) and 20 cows from smaller herds (approx. 100 cows) will al low the cost of poor performance to be estimated sufficiently accurate ly to determine whether further computer-assisted investigation is war ranted. The errors were greatest for the annual culling rate and it is suggested that this be calculated for the whole herd.