The approaches to measurement and evaluation of costs vary along a spectrum of specificity. On one end of the spectrum there is the approach calls for the direct enumeration and costing out of every input consumed in the treatment of a particular patient. On the other end is such gross approach as estimating the cost of an event, for example hospitalisation for heart infarct - by assigning average tariff. The former approach refers to the micro-costing methodology and the more aggregative to the gross-costing methodology. The choice between gross-costing and micro-costing must balance the needs of the analysis - the sensitivity of the results to the bias and precision in the cost estimates - with the difficulty and expense of obtaining a cost estimate.

Micro-costing starts with the detailed inventory and measurement of all inputs consumed in a healthcare intervention. Once the resources consumed have been identified and quantified, they are then converted into values terms to produce a cost estimate. Micro-costing approach is frequently associated with primary data collection within randomised clinical trials or observational research studies.

Gross-costing bases cost estimates on more aggregated information on resource use. It starts with identification of a sequence of "economically significant" events associated with the intervention. These events may include one or more of the following: hospitalizations, physicians services, drugs. Gross cost estimation of the cost of the intervention requires estimating these component event costs, then summing. The processes of measurement and valuation of resources, which are reasonably distinct in micro-costing, are more blurred in gross-costing.

Often these techniques draw on readily available administrative prices. Frequently gross-cost estimates are to be input into larger decision model. The distinguishing features of gross-costing are its simplicity, practicality and its (intended) insensitivity to site-specific details and patient-level characteristics.

The choice of adopted methodology should relate to the type of trial to be conducted. Micro-costing and gross-costing can be used within a single analysis. In general, micro-costing will be more important for aspects of the alternatives under consideration that are likely to diverge in cost, and for intervention and events occurring in the present. Gross-costing is acceptable when using a more exact micro-cost estimate cost will not have an important effect on the analysis. Precision is generally less critical in estimating resources that will be consumed far in the future.

The selection of methodology depends on the individual objectives of the analysis and should be made by the researcher. Nevertheless the choice of a specific methodology should follow standard recommendations and the use of standard sources of data. It should be understood that standardisation of analysis does not apply to the choice of methodology but to the procedures of analysis once the choice is made.

Standard values define the parameters used in the calculation of unit costs. The standard values are for example: gross salary, working hours per year, the number of workable hours of medical staff per year, yearly income in healthcare sector, the average distance of a household to a hospital (to calculate costs of transport), discount rate, inflation rate.

The use of standard values results in the decrease of differences in unit costs estimates between studies. Use of standard values is recommended while presenting analyses for the purpose of reimbursement of new medicines.

The application of standard unit costs has a major role in standardising the economic evaluation of healthcare programs. Despite many well-known limitations, it leads to the elimination of certain differences between trials where different unit costs were used.

The use of standard unit costs is recommended in the case of formal appraisal studies for reimbursement purposes. However for the units with the largest contribution to total or incremental costs, a more detailed costing approach might still be necessary (i.e. direct calculation of unit costs).

The use of the list of standard unit costs is limited to procedures and services for which these costs are calculated. The list of standard unit costs should contain at least standard unit costs for resource items of:

1) out-patient care,

2) in-patient care,

3) pharmaceuticals and medical materials consumed in hospitals,

4) DRG-group (data obtained by large costing studies in which many hospitals take part)

This list will be published and up-dated every year in order to allow adjustments to changes and incorporation of new findings.

In the process of cost calculation the following 5 stages are defined:

1) definition of time horizon and perspective,

2) selection of cost categories,

3) identification of units

4) measuring resource use

5) valuation of resource use

At each stage the choices have to be made between proposed alternatives and these together clearly define the way of proceeding. These choices depend on the aim and specific setting of the study. Since the choices in later stages of the analysis depend on the choices made in previous stages, the stages mentioned above have to be passed in chronological order.

The perspective and time horizon of costing should be the same as the perspective and time horizon of economic evaluation perspective of which cost calculation remains a key part.

The choice of time horizon and perspective is strictly linked to further stages of the analysis in which cost categories selected for the trial are identified and the method of their measurement and evaluation are defined.

The costs are divided into following categories: direct medical and non-medical costs, indirect costs within and outside the healthcare. The selection of cost categories covered in the analysis depends on the perspective has been chosen. For example, a study prepared from a narrow public purchaser would not consider indirect costs or direct costs paid by patient, whereas a study prepared from a broader societal perspective would do so.

According to draft of Polish guidelines for conducting pharmacoeconomic analyses indirect costs outside the healthcare sector (future costs during life-years gained) should be considered in economic evaluation only when they result directly from original intervention. Intangible costs because of the lack of relevant methods are not considered or they are calculated within value of changes in quality of life

The identification of units of resources used raises two questions: what type of resource use are relevant for the disease or intervention studied and to what level of detail do they have to be measured and valued separately. If for example almost all patients who visit a specialist receive the same diagnostic tests, it is not necessary to cost these test separately, but it is better to incorporate the average costs of diagnostic tests into the unit price of an outpatient visit. Only in the case where diagnostic tests performed during consultations vary significantly among patients is it worth identifying these tests as separate cost units.

It is recommended to describe in detail a given procedure in order to define which resources units should be taken into account in the analysis. In order to define the units which have the largest contribution to total and incremental costs it is recommended to conduct a sensitivity analysis. Sensitivity analysis shall also be used to determine which costs should be measured and evaluated in details (using micro-costing methodology) from those which can be evaluated via gross-costing methodology.