Ruggedness defined

Other interesting properties of (bio)chemical sensors are related to their effectiveness for solving real analytical problems. First, these analytical devices should be easy to construct, operate and preserve. Ruggedness, defined here as the confidence that small variations in the experimental conditions (pH, temperature, ionic strength, pressure) will not alter the sensor functioning or response, is very important. A low cost is also desirable, particularly with single-use (bio)chemical sensors. 

Consequently, it was proposed to define (Burns et al. [2005]) Robustness of an analytical procedure is the property that indicates insensitivity against changes of known operational parameters on the results of the method and hence its suitability for its defined purpose and Ruggedness of an analytical procedure is the property that indicates insensitivity against changes of known operational variables and in addition any variations (not discovered in intra-laboratory experiments) which may be revealed by inter-laboratory studies (Burns et al. [2005]). 

The robustness of an analytical method can be defined as a measure of the capability of the method to remain unaffected by small, but deliberate, variations in method parameters. The parameter therefore provides an indication of the method reliability during normal usage. The ruggedness of a method is the degree of reproducibility of test results obtained by the analysis of the same samples under a variety of conditions, such as different laboratories, different analysts, different instruments, different lot of reagents, different days, etc. 

Method validation is defined in the international standard, ISO/IEC 17025 as, the confirmation by examination and provision of objective evidence that the particular requirements for a specific intended use are fulfilled. This means that a validated method, if used correctly, will produce results that will be suitable for the person making decisions based on them. This requires a detailed understanding of why the results are required and the quality of the result needed, i.e. its uncertainty. This is what determines the values that have to be achieved for the performance parameters. Method validation is a planned set of experiments to determine these values. The method performance parameters that are typically studied during method validation are selectivity, precision, bias, linearity working range, limit of detection, limit of quantitation, calibration and ruggedness. The validation process is illustrated in Figure 4.2. 

Defining the specification of the analytical requirement and its solution are difficult and the analytical chemist s experience is vital. Only he or she can accurately predict the ruggedness or vagueness of analytical procedures. Customers know how they would like the automatic instrument to operate and will have a good understanding of the chemistry involved. How a specification should be drawn up is explained in Chapter 2, but unless it is carried out properly the equipment will be over- or under specified or if the problem is not correctly addressed, the automatic equipment will fall into disuse because it fails to achieve the overall objective. 

Define noise, blank, detection limit, signal-to-noise ratio, useful range, ruggedness, selectivity. 

As already mentioned in the introduction, ruggedness is a part of the precision evaluation. Precision is a measure for random errors. Random errors cause imprecise measurements. Another kind of errors that can occur are systematic errors. They cause inaccurate results and are measured in terms of bias. The total error is defined as the sum of the systematic and random errors. 

In a protocol about collaborative studies [10] it is also considered what is called preliminary estimates of precision. Among these the protocol defines the total within-laboratory standard deviation . This includes both the within-run or intra-assay variation (= repeatability) and the between-run or inter-assay variation. The latter means that one has measured on different days and preferably has used different calibration curves. It can be considered as a within-laboratory reproducibility. These estimates can be determined prior to an interlaboratory method performance study. The total within-laboratory standard deviation may be estimated fi-om ruggedness trials [10]. 

Agencies or authorities such as ISO or lUPAC still do not provide any definition of ruggedness. In the chemical literature however, a ruggedness test was defined as [4,12] An intralaboratory experimental study in which the influence of small changes in the operating or environmental conditions on measured or calculated responses is evaluated. The changes introduced reflect the changes that can occur when a method is transferred between different laboratories, different experimentators, different devices, etc. . 

The selection of the factor type of acid in a ruggedness test could be accepted when only the pH is specified by the method rather than the acid used to bring the solution or the buffer up to the desired pH. Clearly, however, in such a case the method is poorly defined. 

Other results obtained from the ruggedness test are the definition of optimized method conditions for the factors and of system suitability criteria for a number of responses. System suitability parameters [6,17] are defined as an interval in which a response can vary for a rugged method. The system suitability criteria are the range of values between which a response (e.g. retention time, capacity factor, number of theoretical plates, resolution) can vary without affecting the quantitative results of the analysis. For instance, a design is performed and the retention time of the main substance varies between 200 s and 320 s without affecting the quantitative determination of the substances. The system suitability criteria for the retention time is then defined as the interval 200 s - 320 s. 

Optimal values for the factors are selected from the tested levels for the factors (extremes or nominal) in function of a number of responses of the method (see also references [16,19]). When one changes the method conditions due to these results one has to be aware that a new method is defined. What is done here is in fact a simplistic way of optimizing a method. The optimization of a method however is a step that is expected to come much sooner in the method development than in the ruggedness testing. One also has to realize that when one defines a new method this requires a new full validation, including a ruggedness test. 

Inserting tolerance intervals of the chromatographic responses in this equation results in rugged intervals for the factors. By comparing these intervals with the inaccuracy of the settings of the experimental conditions a statement about the ruggedness of the method is made. The tolerance intervals of the responses are defined by the experimentator, e.g. 2.5% difference in the area response between two independent analyses is considered acceptable in reference [16], i.e. a value of 0.025 0.307=0.0076 for the above mean response. The rugged interval for the injection temperature is then obtained from equation (29) ... 

Merck also proposed recently an expert system called Ruggedness Method Manager for ruggedness tests of chromatographic assay methods. The system uses fractional factorial designs. Besides the factors to be examined, interactions that possibly also could be relevant have to be defined by the user. The system then calculates a design in which the main effects are not confounded with one of the specified interactions. The interpretation criterion to identify statistically significant effects is not known to the authors of this chapter. 

Other results from a ruggedness test described by some authors are the definition of rugged intervals and of system suitability parameters and the selection of optimal values for the factors. Rugged intervals are defined as the interval between the levels of a factor for which no significant effect is seen on a response [19]. 

Extent of Validation Depends on Type of Method On the one hand, the extent of validation and the choice of performance parameters to be evaluated depend on the status and experience of the analytical method. On the other hand, the validation plan is determined by the analytical requirement(s) as defined on the basis of customer needs or as laid down in regulations. When the method has been fully validated according to an international protocol [63,68] before, the laboratory does not need to conduct extensive in-house validation studies. It must only verify that it can achieve the same performance characteristics as outlined in the collaborative study. As a minimum, precision, bias, linearity, and ruggedness studies should be undertaken. Similar limited vahdation is required in cases where it concerns a fully validated method which is apphed to a new matrix, a well-established but noncol-laboratively studied method, and a scientifically pubhshed method with characteristics given. More profound validation is needed for methods pubhshed as such in the literature, without any characteristic given, and for methods developed in-house [84]. 

Define the following terms instrument precision, injection precision, intra-assay precision, ruggedness, intermediate precision, and interlaboratory precision. 

Earlier guidelines defined precision in terms of system precision and method precision. System precision was the measure of reproducibility based on multiple measurements of a single sample preparation. Method precision was the measure of reproducibility based on analysis of multiple sample preparations. Ruggedness was a measure of day-to-day, analyst-to-analyst, and instrument-to-instrument variation. 

The ICH has broadened and redefined these terms to more accurately describe the method s ability to reproducibly generate analytical results. Precision is defined as a combination of repeatability, intermediate precision, and reproducibility. Repeatability is system precision, as defined previously. Intermediate precision includes multiple analyses by multiple analysts on different days using different equipment within a given laboratory. This is only the first step in demonstrating the ruggedness of the method. 

The process of method transfer must follow a method-transfer protocol which defines the experiments and acceptance criteria necessary to demonstrate the analysts proficiency, equipment s suitability, and true ruggedness of the analytical method. If we assume that any quality analytical laboratory has proficient analysts who operate suitable equipment, then the method transfer stands as an ongoing means to substantiate the suitability of the original method validation. Example 5 contains an example of a method-transfer protocol for a chromatographic procedure. 

Ruggedness/robustness Defined based on an experimental design and data (sensitive parameters and a range for each parameter in the final test method)... 

The robustness of an analytical method can be described as the ability to reproduce the method in different laboratories or under different circumstances without the occurrence of unexpected differences in the obtained results. The term ruggedness is considered here as a synonym for robustness. The robustness of a method is tested in a robustness test. The most frequently u.sed definition for robustness in this area is due to the International Conference on Harmonisation of Technical Requirements for the Registration of Pharmaceuticals for Human Use (ICH) [79,80. It defines robustness as follows. The robustness of an analytical procedure is a measure of its capacity to remain unaffected by small, but deliberate variations in method parameters and provides an indication of its reliability during normal usage."... 

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