Although the criteria from which one can assess the homogeneity of the environment in which health centers they operate are different, the analysis should only be considered which affect the possibilities and requirements of production. In this sense, given that market conditions are the same (only discusses public health facilities) and the different administrative unit to which the center is the differential element on which the present work, the only aspect of the environment, affecting production requirements, it remains to assess the general health status of the population covered. Thus, this means ultimately is to ascertain whether, in general, schools that constitute the sample under study or not facing the same problem (or pattern) of health.

To check whether this is so, since the minimum level of disaggregation of data is the province, assessing the health standard of all the Spanish provinces taking into account the state of health of a population can not be judged solely through its life expectancy and mortality and morbidity, and it depends on many different factors as behavior and lifestyle of the individual, or to external factors such as the provision of health services or the socio-economic and environment in which desarrolla7 8.

Applying a cluster analysis of provincial data, by quantifying the various factors that influence the health of an individual, describe the pattern of health of the Spanish provinces (Table 1) shows that there are different patterns of health Spanish population (two in particular) and the three provinces under analysis shared with 19 others, the same standard of health (Table 1).

Verified in this way, the homogeneity of the environment, the next step, as proposed by Charnes et al, is to verify that the health facilities under study are homogeneous products and recursos6.

In this regard, the General Health Law (1986) states that all health centers to develop activities aimed at promotion, prevention, cure and rehabilitation of the health of the inhabitants of the area of basic health and to this end, All are equipped with the personnel and material necessary to fulfill these functions (art. 63). Although, as this does not appear that there may be significant differences in either the product offered by the centers or the factors used to produce it, the Act recognizes a demand factor (the population assigned to each site) that affects the production and the allocation of resources and can lead differences between centers. For this reason, it was decided to eliminate from the sample all the centers that belong to the selected area as homogeneous in terms of health, only offered, given the demographics of its population, services in general medicine. Also, for the same reason it was decided not to consider in assessing the efficiency of facilities providing on site services specialist support, or activities or resources related to these services.

Both decisions provide a sample of 66 health centers homogeneous, of which 14 belong to the region of Álava, 11 and 41 are Rioja, Navarre.

The empirical specification of the problem

The requirements of the EDA regarding the empirical specification of the problem affects the choice of mathematical form of the model as the selection of variables. Two decisions that have as their ultimate goal of the design envelope data model that best fits the reality of production under consideration.

Productive organizational characteristics of health centers and above the operational benefits of information and models envelopes warrant that, among all the models available in the actualidad9 is BCC10 choose a model to estimate a production frontier to assess, from the standpoint of output, the pure technical efficiency of health facilities in the sample. This approach will be considered inefficient those centers that are not capable of output to maximize the available resources. Events like the low power that managers and health professionals have to modify and identify many of the resources used in the production of primary care, or the value that people obtain grants itself the output provided by the health center, explain that the criteria for assessing the (in) efficiency is maximizing output, rather than minimizing the inputs, as usual in the analysis of productive efficiency.

The selection of variables that give content to both the inputs and outputs is therefore another key study decisions, decision, which, like its predecessors, will also be influenced by the manufacturing and information centers selected health.

Thus, in the absence of comprehensive and uniform approximations of the product offered and coordinated information systems which provide a minimum set of data with which, as some authors have intentado11-14, try to adjust the size of the product to differences in quality and / or severity, there was recourse, as in most jobs anteriores15-20, use the number of consultations as an approximation of the proceeds for the health center. The ability to differentiate nursing from medicine and general pediatrics allowed in any way consider the qualitative differences in the query.

Consequently, three variables were selected as representative of the output provided by the health center:

Y1: Number of general medical consultations per year (in thousands).

Y2: number of pediatric consultations a year (in thousands).

Y3: number of nurses per year (in thousands).

The analysis of the budget distribution of health facilities has helped, on its part, the selection of variables representing the resources used. The relative weight of expenditure reveals the personal human capital-intensive primary care services and justifies the selection of the number of physicians, paediatricians and nurses in full-time (or equivalent) as determinants of the service. Alongside them, and as the analysis of the budget for schools, selecting a variable representative of the consumption of material resources: the spending, in millions of pesetas, flows of goods and services. In summary, the selected variables as determinants of the proceeds from the 4 primary care were:

X1: doctors full-time or equivalent.

X2: pediatricians or full-time equivalent.

X3: nurses as full-time or equivalent.

X4: expenditure on goods and services flows (in millions).

As with the product, the variables selected as indicators of the resources used in production are those that generally have been used in studies that have analyzed the efficiency of primary care in connection estricto11-13.

The information on the 7 selected variables (4 inputs and 3 outputs) refers to the year 1997 and was supplied by the Directorate of the District of Álava Basque Health Service (Osakidetza), Management of Primary and Rioja Programming and Education Unit of the Directorate of Primary Care and Mental Health Services Health Navarro (Osasunbidea). In Table 2 indicates some of the most representative descriptive statistics of these variables.