Commission Regulation N° 3127/94, article 1, it is stated that a prediction
formula should be based on « ... a representative sample of the national
or regional pigmeat production concerned by the assessment method... ».
National pig populations are generally very heterogeneous and include several
sub-populations, such as different sexes and breeds. Moreover selection concerns
carcasses and the carcasses can only be selected in slaughterhouses. The main
problem is therefore how to choose slaughterhouses and to select carcasses in
these slaughterhouses in order to get a representative sample?
Furthermore the most popular statistical method used for the assessment of pig classification methods is linear regression. But it is a well-known fact from regression theory that a wider range of x-values (prediction variables) will give more accurate estimates of the formula coefficients (intercept a and slope b). This is contradictory with a representative sample, which can be understood as at random. This means that it has to be clearly defined what should be a representative sample for the assessment of pig classification methods.
Finally non-linear statistical methods have also been used. Sampling influence on the estimates is still not clear.
used until now
different approaches have been used in EU to deal with these sub-populations.
Sub-populations are ignored.
Sub-populations are ignored, except that the
total numbers of animals for some sub-populations are fixed beforehand and
chosen proportional to the numbers in the population.
· Sub-populations are taken into account using one sub-sample for each sub-population considered.
strategies have also been used to select carcases on the predicting variables :
No selection on potential predicting variables.
Selection on potential predicting variables with
introduction of these variables in the formula.
Selection on potential predicting variables
without introduction of these variables in the formula.
· Selection on variables correlated with the predicting variables.
main objective is to define clearly how a representative sample can be chosen
for assessing pig classification methods. Among the different approaches used
for dealing with, on one hand sub-populations, and on the other hand, the
selection on potential predicting variables, it has to be determined which ones
objective is to define the sampling plans for WP1 and WP3.
sampling plans for WP1 and WP3 have been designed. Model for testing operator
effect on classification measurements is still under discussion.
questionnaire for describing national pig populations has been sent to all EU
Member States and also to the Newly Associated States. Compilation of
informations is running.
documents have been written about sampling for instruments with a moderate
number of carcass measurements :
works are still running about consequences of selection on estimated lean meat,
combination of global oversampling and quotas by subpopulations, sampling for
instruments with a large number of carcass measurements.