Estudo comparativo entre métodos de controle quantitativo da produção leiteira

Resumo

This paper deals with the estimation of milk produ- ction by means of weekly, biweekly, bimonthly observations and also by the method known as 6-5-8, where one obser- vation is taken at the 6th week of lactation, another at the 5th month and a third one at the Sth month. The data studied were obtained from 72 lactations of the Holstein Friesian breed, of the “Escola Superior de Agricultura “Luiz de Queiroz” (Piracicaba, S.P., Brazil), being 6 calvings on each month of the year and also 12 first calvings, 12 second calvings, and so on, up to the sixth.

The authors criticize -the use of “maximum error to be found in papers dealing with this subject, and also the use of mean deviation. The former is completely superseded and nadvisable and the latter, although equivalent, to the usual standard deviation, has only 87.6%, of its efficiency, accor- ding to KENDALL (9,pp. 130, 131,10,pp.6-7). 

The data obtained were compared with the actual production, obtained by daily control and the deviations observed were studied. Their means and standard deviations are given on the table IV.

In spite of BOX’s recent results (11) showing that with equal numbers in all classes a certain inequality of variances is not important, the methods, before carrying out the analysis of variance, thus avoiding to put together methods with too different standard deviations, We compared the three first methods, to begin with (Table VI). Then we caried out the analysis with the four first methods. (Table VII).

These analyses of variance compare the arithmetic means of the deviations by the methods studied, and this is equivalent to compare their biases. So we conclude that season of calving and order of calving do not effect the biases, and the methods themselves do not differ from this viewpoint, with the exception of method 6-5-8.

We denote above by one asterisk significance at the 5% level, and by three asterisks when at the 0.19, level of probability.

In this way we should conclude that the weekly me- thods of control show a significant positive bias, which, however, is less than 0.5%. The 6-5-8 method is proved again to be positively biased, and here the bias equals 5.9% of the mean milk production. The monthly and bimonthly methods of control may be assumed to be unbiased.

The precision of the methods studied may be judged by their standard deviations or by intervals covering, with a certain probability (95%, for example), the deviation x corresponding to an estimate obtained by one of the methods studied. Since the difference x- x, where x is the mean of the 72 deviations obtained for each method, has a + distribution with mean zero and estimate of standard deviation and the limit of t for the 59, probability level with 71 de- grees of freedom is 1.99, then the interval to be considered is given by. 

The intervals thus calculated are given on the table IX.