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     The reasons that motivate the clinical usage of scales, questionnaires, tests and rates can be diverse like help for diagnostic, support for the frame of medical treatment, communication with other professionals, and establishment of a populational point of reference.
     The "quantitative" urge that has ruled the health disciplines during many years has converted the clinical measurement into an omnipresent activity for the health practice and study. Based on this approach, the legitimacy of the numerical data has been backed up by the physic and objectifiable world from where it comes: blood pressure, volume of cardiac ejection, body mass index. Nevertheless the health attention, following its aspiration of providing answers to the complex human dimensions from a growing multidisciplinarity, has forced to explore "numerical statements" related to how much of an attribute is present in one person. E.g.: social support perceived or quality of life of a person.¹ This kind of attributes is what is know as construct on its metric analysis, meaning a hypothetic concept whose existence could never be totally confirmed.¹
     It is necessary to mention that patients and professionals use scales constantly to communicate between each others, such as "I have a headache" or "I have lost some weight". However, these measurements must be considered as a process of evaluation and personal discernment². On the other hand, a measurement scale must be adapted to a set of measurement axioms (model) to explain the quality of the ratio between its variables and, if possible, the adjustment of the data to the predictions of the model¹. Based on that, a scale of risk of pressure ulcer must be made of variables interrelated and based on the model established (e.g.: humidity, mobility.); the scores of that scale should be able to predict the apparition of pressure ulcers.
     These metric properties are precisely the ones that differentiate a personal judgment from a scale of measurement. However, depending on the discipline, these properties will be studied in the field of psychometry, econometrics, anthropometry, or where appropriate, in the field of clinimetrics as the measurement of clinical phenomena was firstly denominated by Alvan R. Feinstein².
     In the process of clinimetric construction, the fundamental properties demanded from a scale of measurement are the validity and the reliability, in other words, the standardization of the contents and the standardization as scale of measurement, respectively². The validity denotes the scientific utility of the scale; it denotes to what extent, what has been measured approaches what was intended to measure³. On its side, the reliability points out the scientific utility of the scale as a tool that shows its accuracy when measuring, putting to the test its reproducibility in measurements performed later or by different observers³.
     After a long and complex scientific process of elaboration of a clinical scale, the exposition of which surpasses the content of the present text, the scale is put at the health professional's disposal. He will be the one in charge of deciding its use depending on the utility perceived. Among these utilities that are found complementarily it can be mentioned:

Help to diagnosis. The clinical diagnosis is not determined by the test that only warns about the risk of suffering the problem. It will be the professional the one in charge of confirming or not its presence, using also other clinical data taken into account as well. Great importance is attached to the fact of knowing the capability of the methods of diagnosis and prognosis used for the correct interpretation of the results. In order to achieve it, it is fundamental to take into account the sensitivity and specificity of a scale summed to the expected prevalence of the problem in the population under study. It must be remembered that the professional criterion is always necessary to take a clinical decision; the scale should not make the decision for the professional.
     As an explanatory remark, the concepts of sensitivity and specificity should be defined. The sensitivity corresponds to the proportion of cases (persons with a health problem) for which the test (scale) has a positive result, that is to say, it points out the capability of the scale to detect cases. On the other side, the specificity is the proportion of no-cases for which the test result (scale) is negative, in other words, it measures the capability to detect no-cases.⁴

Support for the threshold of treatment. If the final aim for the administration of a scale is to determine a therapeutic attitude, it will be necessary to define in advance a threshold of treatment, that is to say, a probability of suffering from the health problem previous to the administration of the scale that will trigger the decision of treating the patient. This threshold probability depends on the benefit of treating persons having the problem, and also depends on the cost of treating persons without the problem⁴. If the benefit of intervention is high and the costs are low, the threshold of treatment will be low as it happens in the case of pressure ulcers, and vice versa. Again, the professional criterion is revealed as determinant element to make clinical decisions beyond the numeric value obtained after the administration of a scale.

Communication with other professionals. The value obtained as a result of the administration of a scale allows a synthesized and objective communication, avoiding misunderstandings at the receptor side. This utility is widely accepted for the communication between different medical attendance levels. It must be mentioned that the value found in a scale can and should be tinged with information of interest, that might not have been caught in the scale, or that might be clinically relevant.

Establishment of a populational referent.  Being the scale a tool of measurement whose clinimetric properties are built by means of a survey made on a concrete population, it is logical to think that the value of each person under research is located relatively regarding the value of the rest of the population.
     - In this way, a cut-point established in the scale can serve as criterion for the distribution of resources or for derivation to another professional. Among the mentioned resources we can point out the anti-slough mattresses or the walkers; among the professionals to whom the patient will be derived we can find the psychologists or the rehabilitator. At this point, it is fundamental the role of the professional. He has to attenuate the imperfections and the injustices of any criterion, whose external nuances escape from its coverage.
     - Another important utility is found in the clinical research. When finding objective and quantifiable values with a populational referent, the scale becomes one of the most used methods of data collection in the quantitative research. In this way, using a scale, investigations to describe populational attributes can be developed; those populational attributes can be compared with other populations or within the same population mediated by a time interval, or they can even be used to value the effectiveness of a certain intervention. The scales of quality of life are the paradigm of this usage and have made it possible to expand the analysis of effectiveness from the point of view cost-utility. Nevertheless, some gaps from the point of view of the economic and utilitarian models still need to be solved.
     As an added limitation to the extrapolation of instruments developed in contexts where populations with socio-cultural differences exist, the transcultural adaptation to the environment where they are going to be used is a mandatory step many times underestimated in the works of validation. As a result of this negligence it is frequent to find a distortion or damage in the validity of the scale content, and a variation in its sensitivity and specificity.
     It must not be forgotten that there is not perfect test, and that any score observed in a questionnaire is the result of adding a random error to the real score¹. The disregard of the measurement error in the mentioned tests might put the person under analysis at risk. The process of clinimetric construction might, if it is done correctly, diminish this error to the minimum. The critical evaluation of the instrument and the analysis of the validation will contribute with enough factors to determine its applicability.
     Finally, it is necessary to remind that the indiscriminate usage of clinimetric instruments will never replace the essential abilities needed for the clinical interview nor the qualitative knowledge of the human answer at the moment of bringing in a judgment. It is in synergy with these abilities and knowledge, that the clinical deliberation is increased and enriched.

References

1. Nunnally J, Bernstein I. Teoría Psicométrica. 3ª Edición. México: Mc Graw Hill, 1995.
2. Feinstein AR. Clinimetrics. New Haven: Yale University Press, 1987
3. Streiner DL, Norman GR. Health measurement scales. A practical guide to their development and use. 3rd Edition. New York: Oxford University Press, 2003.
4. Latour J. Análisis de decisiones. Quaderns 12. Valencia: Instituto Valenciano de Estudios en Salud Pública, 1997.

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