Comparison of different statistical approaches used to evaluate the performance of participants in a proficiency testing program

Five statistical approaches were applied for assessing the performance of participants in 19 rounds(2007-2011) of the Proficiency Testing Program for lead in blood determination (PEP-Pbs), conducted by Instituto Adolfo Lutz. The performance evaluation was provided by using the z-score. The following statistical approaches were tested: 1 – mean and standard deviation, after rejecting outliers; 2 – median and normalized inter-quartile range; 3 – robust mean and robust standard deviation; 4 – robust meanand standard deviation for proficiency assessment of 3 μg/100 mL (for concentrations up to 40 μg/100mL and a standard deviation for proficiency assessment of 7.5 % of the assigned value (for concentrations above 40 μg/100 mL); 5 – robust mean and standard deviation for proficiency assessment of 2 μg/100 mL(for concentration up to 40 μg/100 mL) or 5 % of the assigned value (for concentrations above 40 μg/100mL). The approach 4 showed to be the most adequate statistical methodology to assess the performance of participating laboratories in the PEP-Pbs. (AU) Cinco abordagens estatisticas foram aplicadas para avaliar o desempenho dos participantes em 19 rodadas(2007-2011) do Programa de Ensaio de Proficiencia para determinacao de chumbo em sangue (PEP-Pbs),organizado pelo Instituto Adolfo Lutz. A avaliacao de desempenho foi realizada utilizando-se o indice-z, eas seguintes abordagens estatisticas foram testadas: 1 - media e desvio padrao, apos a exclusao dos valoresdispersos; 2 - mediana e intervalo interquartil normalizado; 3 - media robusta e desvio padrao robusto;4 - media robusta e desvio padrao de proficiencia de 3 μg/100 mL (para concentracoes de ate 40 μg/100mL) e desvio padrao de proficiencia de 7,5 % do valor designado (para concentracoes superiores a 40μg/100 mL); 5 - media robusta, desvio padrao de proficiencia de 2 μg/100 mL (para concentracoes de ate40 μg/100 mL) e desvio padrao de proficiencia de 5 % do valor designado (para concentracoes acima de40 μg/100 mL). A abordagem 4 mostrouser mais adequada para avaliar o desempenho dos laboratoriosparticipantes do PEP-Pbs.(AU)