Combining the allowable accuracies of the calibrator/control and the osmometer provide the expected range of the mean for a series of tests. The osmometer repeatability specification is not considered when determining this combined accuracy.
For example, assume that the specifications are:
Formulation tolerance of 290 mOsm/kg H2O (nominal) control solution:
±2 mOsm/kg H2O
Osmometer linearity specification: ±1%
Converting the linearity percentage to a mOsm/kg H2O value at the
nominal value of 290 mOsm/kg H2O gives:
±2.9 mOsm/kg H2O
Thus the combined accuracy of the control solution and the
±2 mOsm/kg H2O + ±2.9 mOsm/kg H2O = ±4.9 mOsm/kg H2O
Therefore the acceptable range of values for the mean of a set
of results is:
290 mOsm/kg H2O ±4.9 mOsm/kg H2O, or 285.1 to 294.9 mOsm/kg H2O.
Now consider the osmometer repeatability. If the osmometer repeatability
specification is stated in terms of a standard deviation and has a
1s or 1 S.D. = 2 mOsm/kg H2O
and the actual standard deviation calculated from a set of data is ≤ 2 mOsm/kg H2O, then the osmometer is performing acceptably from a repeatability standpoint.
Using the acceptable range of values for the mean from above and a 1 standard deviation value for the repeatability, the acceptable range of results for any individual sample tested within the data set is:
Lower range limit: 285.1 - 2 = 283.1 mOsm/kg H2O
Upper range limit: 294.9 + 2 = 296.9 mOsm/kg H2O.
Using a repeatability value equal to 2 standard deviations, the acceptable range of results for any individual sample tested within the data set is:
Lower range limit: 285.1 - (2 X 2) = 281.1 mOsm/kg H2O
Upper range limit: 294.9 + (2 X 2) = 298.9 mOsm/kg H2O.
Therefore, with this osmometer and testing this calibrator/control, if the mean of a series of test results lies between 285.1 and 294.9 mOsm/kg H2O and the standard deviation of that group of test results is ≤ 2 mOsm/kg H2O, then the complete system (i.e. calibrator/control and the osmometer) is performing acceptably with an acceptable range of results as determined above.