1212569_21823227.jpg eu_flag_tempus TEMPUS project NETREL learing course Ruggedness, robustness, stability Šimon Vojta and Branislav Vrana RECETOX, Masaryk University, Brno, CR • 1212569_21823227.jpg eu_flag_tempus TEMPUS project NETREL learing course Ruggedness Testing •the degree of reproducibility of results obtained under a variety of conditions, such as different laboratories, analysts, instruments, environmental conditions, operators and materials •a measure of reproducibility of test results under normal, expected operational conditions from laboratory to laboratory and from analyst to analyst •determined by the analysis of aliquots from homogeneous lots in different laboratories 1212569_21823227.jpg eu_flag_tempus TEMPUS project NETREL learing course Ruggedness testing •by considering each effect separately, by repeating measurements after varying a particular parameter by a small amount (say 10%) and controlling the other conditions appropriately •this can be labour-intensive as a large number of effects may need to be considered •for a well-developed method, most of the effects can be expected to be small, it is possible to vary several parameters at the same time •any stable and homogeneous sample within the scope of the method can be used for ruggedness-testing 1212569_21823227.jpg eu_flag_tempus TEMPUS project NETREL learing course Plackett–Burman design of ruggedness testing •7 parameters, each tested at two levels (low-high) e.g. –1. volume of sample –2. time of extraction –3. flow rate –4. temperature –5. pH –6. salt concentration –7. modifier addition 128 possible combinations! The Plackett–Burman design allows seven independent factors to be examined in eight experiments. Factors that might be thought to influence a result could include, acid concentration, time of extraction, flow rate, temperature, etc. If one can identify two levels of each of the parameters under investigation, there are 128 combinations that can be written out. 1212569_21823227.jpg eu_flag_tempus TEMPUS project NETREL learing course Plackett–Burman design of ruggedness testing •The design described allows information to be gathered from only eight experiments •A, B, C, D, E, F and G – nominal level •a, b, c, d, e, f and g - alternative level •The chosen levels may be the extreme values of the parameter, e.g. the two extremes of temperature likely to be encountered during use of the method 1212569_21823227.jpg eu_flag_tempus TEMPUS project NETREL learing course Plackett–Burman design of ruggedness testing To find if changing factor ‘A’ to ‘a’ has an effect, A is calculated In Table, you will see that the selected eight combinations of these letters leads to a balance between the to be used when running the eight experiments. The results from the experiments are shown as l, m, p, w, v, x, y and z. For each factor, the difference is calculated between the average of the results obtained with the factor at its nominal value and the average of the results obtained with the factor at its alternative value. 1212569_21823227.jpg eu_flag_tempus TEMPUS project NETREL learing course Plackett–Burman design of ruggedness testing With this combination the effect of the other factors cancels out! 1212569_21823227.jpg eu_flag_tempus TEMPUS project NETREL learing course Plackett–Burman design of ruggedness testing Changing factor ‘B’ to ‘b’ is examined by calculating B, as follows: 1212569_21823227.jpg eu_flag_tempus TEMPUS project NETREL learing course Plackett–Burman design of ruggedness testing •The next step is to arrange the seven differences, A to G, in numerical order (ignoring the sign). •To calculate if any of the differences are statistically significant, a statistical test (t -test) is applied. •Equation below is used to compare the difference |Di | with the expected precision of the method, s and the level of confidence used (t-test). • • •For cases where equation is true, the change from the nominal to the alternative level is significant. • 1212569_21823227.jpg eu_flag_tempus TEMPUS project NETREL learing course Plackett–Burman design of ruggedness testing •NOTE! the results of the test will be misleading if the factors investigated are not independent •Such a study may be used to set the level of control that should be applied at particular stages of the method, e.g. adjust the pH to 6.5 ± 0.2 •It is also possible to study the effect of potential interferences by using this approach