**4.1 Clearly define the measurand**

It is very important that the customer and the toxicologist have a clear understanding of exactly the property being measured. Interpretation will then be applied to a specific measurand in a specific context where FFP can be appropriately determined. For our example we will assume that the measurand is the venous whole blood alcohol concentration collected from a specific individual at a specific time and location.

#### **4.2 Identify the measurement function**

We will assume the following basic model for our measurement of blood alcohol concentration (BAC) by headspace gas chromatography:

$$\mathbf{C}\_{corr} = \frac{\mathbf{C}\_0 \,\mathrm{R}}{\overline{\mathbf{X}}} \cdot f\_{dilutor} \tag{14}$$

where: Ccorr = the corrected BAC results

C0 = the mean of the original measurement results

R = the traceable reference control value

*X* the mean results from measuring the controls

fdilutor = the correction factor for the dilutor

Equation 14 is a basic multiplicative model that includes four components of uncertainty and corrects for analytical bias.
