DEST - Departamento de Estatística

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Choosing between Cox proportional hazards and logistic models for interval-censored data via bootstrap
(Journal of Applied Statistics, 2003) Corrente, José Eduardo; Chalita, Liciana V. A. S.; Moreira, Jeanete Alves
This work develops a new methodology in order to discriminate models for interval-censored data based on bootstrap residual simulation by observing the deviance difference from one model in relation to another, according to Hinde (1992). Generally, this sort of data can generate a large number of tied observations and, in this case, survival time can be regarded as discrete. Therefore, the Cox proportional hazards model for grouped data (Prentice&Gloeckler, 1978) and the logistic model (Lawless, 1982) can be fitted by means of generalized linear models. Whitehead (1989) considered censoring to be an indicative variable with a binomial distribution and fitted the Cox proportional hazards model using complementary log-log as a link function. In addition, a logistic model can be fitted using logit as a link function. The proposed methodology arises as an alternative to the score tests developed by Colosimo et al. (2000), where such models can be obtained for discrete binary data as particular cases from the Aranda-Ordaz distribution asymmetric family. These tests are thus developed with a basis on link functions to generate such a fit. The example that motivates this study was the dataset from an experiment carried out on a flax cultivar planted on four substrata susceptible to the pathogen Fusarium oxysoprum. The response variable, which is the time until blighting, was observed in intervals during 52 days. The results were compared with the model fit and the AIC values.
Properties of the weighting cell estimator under a nonparametric response mechanism
(Survey Methodology, 2004-06) Silva, Damião Nóbrega da; Opsomer, Jean D.
The weighting cell estimator corrects for unit nonresponse by dividing the sample into homogeneous groups (cells) and applying a ratio correction to the respondents within each cell. Previous studies of the statistical properties of weighting cell estimators have assumed that these cells correspond to known population cells with homogeneous characteristics. In this article, we study the properties of the weighting cell estimator under a response probability model that does not require correct specification of homogeneous population cells. Instead, we assume that the response probabilities are a smooth but otherwise unspecified function of a known auxiliary variable. Under this more general model, we study the robustness of the weighting cell estimator against model misspecification. We show that, even when the population cells are unknown, the estimator is consistent with respect to the sampling design and the response model. We describe the effect of the number of weighting cells on the asymptotic properties of the estimator. Simulation experiments explore the finite sample properties of the estimator. We conclude with some guidance on how to select the size and number of cells for practical implementation of weighting cell estimation when those cells cannot be specified a priori.
Testing homogeneity in Weibull-regression models
(Biometrical Journal, 2005) Bolfarine, Heleno; Valença, Dione M.
In survival studies with families or geographical units it may be of interest testing whether such groups are homogeneous for given explanatory variables. In this paper we consider score type tests for group homogeneity based on a mixing model in which the group effect is modelled as a random variable. As opposed to hazard-based frailty models, this model presents survival times that conditioned on the random effect, has an accelerated failure time representation. The test statistics requires only estimation of the conventional regression model without the random effect and does not require specifying the distribution of the random effect. The tests are derived for a Weibull regression model and in the uncensored situation, a closed form is obtained for the test statistic. A simulation study is used for comparing the power of the tests. The proposed tests are applied to real data sets with censored data.
Testing homogeneity inWeibull error in variablesmodels
(Wiley, 2006) Valença, Dione Maria; Bolfarine, Heleno
Wediscusspropertiesofthescorestatisticsfortestingthenullhypothesis ofhomogeneityinaWeibullmixingmodelinwhichthegroupeffectismodelledas arandomvariableandsomeofthecovariatesaremeasuredwitherror.Thestatistics proposed are based on the corrected score approach and they require estimation onlyundertheconventionalWeibullmodelwithmeasurementerrorsanddoesnot require that the distribution of the random effect be speciﬁed. The results in this paperextendresultsinGimenez,Bolfarine,andColosimo(AnnalsoftheInstitute ofStatisticalMathematics,52,698–711,2000)forthecaseofindependentWeibull models.Asimulation study is provided.
An alternative model for on-line quality monitoring for variables
(International Journal of Production Economics, 2007) Ho, Linda Lee; Medeiros, Pledson Guedes de; Borges, Wagner
An alternative model for an on-line monitoring process for variables is presented in this paper. The characteristic of interest follows a normal distribution and changes of the distribution parameters are described by a two-state Markov chain. Items are produced independently and a single item is examined at every m manufactured items similar to an on-line quality procedure to monitor process for variables previously presented by Taguchi et al. [1989. Quality Engineering in Production Systems. McGraw-Hill, New York]. The objective of this study is to determine the optimal sampling interval m and the optimal control limit d that minimizes the expected cost function. A program using S-PLUS was developed to compare Taguchi’s and the alternative proposal. The alternative model provides a better performance than Taguchi’s, since it yields a lower cost and feasible parameters (control limits and sampling interval).
Prevalência de transtornos mentais comuns e avaliação da qualidade de vida no climatério
(Revista da Associação Médica Brasileira, 2007-07) Galvão, Lílian Lira Lisboa Fagundes; Farias, Maria Cláudia Saldanha; Azevedo, Paulo Roberto Medeiros de; Vilar, Maria José Pereira; Azevedo, George Dantas de
OBJETIVO. Avaliar a prevalência de transtornos mentais comuns e a qualidade de vida (QV) relacionados à saúde em mulheres no climatério. MÉTODOS. Foram incluídas 191 mulheres (entre 45 e 65 anos) em estudo analítico transversal. Um questionário, contendo informações pessoais, hábitos/saúde e dados demográficos, foi administrado em associação a instrumentos validados para medir qualidade de vida (SF-36, Medical Outcomes Study 36-item Short-Form Health Survey) e estimar a prevalência de transtornos mentais comuns (SRQ-20, Self Reporting Questionnaire). RESULTADOS. Com a utilização do SRQ-20 e empregando ponto de corte de oito ou mais respostas afirmativas, 39,8% das mulheres foram classificadas como apresentando transtornos mentais comuns. Evidenciou-se maior prevalência de transtornos mentais comuns e piores escores de QV nas mulheres com pouca escolaridade, baixa renda familiar e que não exerciam atividades profissionais fora do domicílio. Os escores médios para todos os domínios do SF-36 foram significativamente mais baixos nas mulheres categorizadas por apresentarem transtornos mentais comuns. CONCLUSÃO. A prevalência de transtornos mentais comuns é elevada na amostra de mulheres no climatério e está associada com repercussões negativas sobre sua qualidade de vida. Fatores psicossociais exercem significativa influência, e estratégias de suporte psicológico deveriam ser instituídas no contexto das políticas de saúde voltadas para mulheres no climatério.
Human papillomavirus infection in women attendes at a cervical cancer screening service in Natal, Brazil
(Brazilian Journal of Microbiology, 2008-07) Fernandes, Thales Allyrio Araújo de Medeiros; Meissner, Rosely de Vasconcellos; Bezerra, Laelson Freire; Azevedo, Paulo Roberto Medeiros de; Fernandes, José Veríssimo
We analyzed cervical specimens of 202 women, aged 15 to 64 years, attended at Luis Antonio Hospital, Natal, Brazil, to determine the prevalence of HPV and identify the more frequent genotypes and risk factors for HPV nfection in women attended at a cervical cancer screening service. Two specimens were collected from each patient: one for cytological examination and the other to detect HPV DNA by PCR, and typing by dot blot hybridization. A total of 54.5% of the sample had normal cytology and 45.5% had cytological alterations. HPV was detected in 24.5% of the cytologically normal women and in 59.8% of those with altered cytology. Both single and double HPV infection increased the likelihood of cytological alterations. Thirteen types of HPV were identified, most of which were high risk. HPV 16 was the most prevalent single-type infection, followed by HPV 58. The most frequent double infection was the association between HPV 56 and 57. The prevalence of HPV in cytologically normal women was greater than that reported for countries on all the continents except Africa. The inverse was observed in women with cytological alterations. The distribution of HPV types was similar to that described for the Americas, with some differences. Multiple sexual partners was the only risk factor showing an association with the presence of HPV infection.
Prevalence of HPV infection by cervical cytologic status in Brazil
(International Journal of Gynecology and Obstetrics, 2009) Fernandes, José Veríssimo; Meissner, Rosely de Vasconcellos; Carvalho, Maria Goretti Freire de; Fernandes, Thales Allyrio Araújo de Medeiros; Azevedo, Paulo Roberto Medeiros de; Villa, Luisa Lina
Objectives: To assess the prevalence of human papillomavirus (HPV) infection according to cervical cytologic status in northeastern Brazil; identify other risk factors for low- and high-grade squamous intraepithelial lesions (LSILs and HSILs); and identify the most prevalent HPV genotypes associated with the lesions. Method: Two cervical smears were collected from 250 women referred for cancer screening, one for cytologic examination and the other to test for the presence of HPV by PCR with genotyping by dot blot hybridization. Result: There were 110 healthy cervices, 82 LSILs, and 58 HSILs. The overall HPV prevalence was 48%, with higher rates for HSILs, and HPV-16 was the most prevalent type. Age, multiple sexual partners, type of HPV present, smoking, and early onset of sexual activity were risk factors for cervical lesions. Conclusion: Age, multiple sexual partners, and infection with HPV-16 increased the risk of having LSILs or HSILs. Early onset of sexual activity and smoking only increased the risk of having HSILs.
Assessment of personal hygiene and practices of food handlers in municipal public schools of Natal, Brazil
(Elsevier, 2009) Campos, Ana Karina Couto; Cardonha, Ângela Maria Soares; Pinheiro, Liana Bacurau Galvão; Ferreira, Neide Rocha; Azevedo, Paulo Roberto Medeiros de; Stamford, Tânia Lúcia Montenegro
The aim of this study was to assess the hygiene practices of food handlers in municipal schools of Natal, Brazil, where 27 public schools were evaluated, using a checklist and microbiological analysis of hands. It was found that 74.1% of the handlers did not receive periodic training, 51.9% did not undergo annual health examinations and 100% did not practice proper hand hygiene, a situation that reflected significantly (p < 0.05) in hand contamination, in which fecal coliforms were detected on 55.6% of the hands analyzed. It was concluded that the schools studied did not have appropriate hygienic conditions, suggesting the need for interventions that ensure the quality of school food served to the children
Conhecimentos, atitudes e prática do exame de Papanicolaou por mulheres, Nordeste do Brasil
(Scielo, 2009-02) Fernandes, José Veríssimo; Rodrigues, Silvia Helena Lacerda; Costa, Yuri Guilherme Alexandre Silva da; Silva, Luiz Cláudio Moura da; Brito, Alípio Maciel Lima de; Azevedo, Judson Welber Veríssimo de; Nascimento, Ermeton Duarte do; Azevedo, Paulo Roberto Medeiros de; Fernandes, Thales Allyrio Araújo de Medeiros
OBJETIVO: Analisar conhecimentos, atitudes e práticas das mulheres em relação ao exame citológico de Papanicolaou e a associação entre esses comportamentos e características sociodemográfi cas MÉTODOS: Inquérito domiciliar com abordagem quantitativa. Foram entrevistadas 267 mulheres com idade de 15 a 69 anos, selecionadas de forma estratifi cada aleatória, residentes no município de São José do Mipibu, RN, em 2007. Utilizou-se questionário com perguntas pré-codifi cadas e abertas, cujas respostas foram descritas e analisadas quanto à adequação dos conhecimentos, atitudes e prática das mulheres em relação ao exame preventivo de Papanicolaou. Foram realizados testes de associação entre as características sociodemográfi cas e os comportamentos estudados, com nível de signifi cância de 5%. RESULTADOS: Apesar de 46,1% das mulheres entrevistadas terem mostrado conhecimento adequado, proporções de adequação signifi cativamente maiores foram observadas em relação às atitudes e prática quanto ao exame: 63,3% e 64,4%, respectivamente. O maior grau de escolaridade apresentou associação com adequação dos conhecimentos, atitudes e prática, enquanto as principais barreiras para a realização do exame relatadas foram descuido, falta de solicitação do exame pelo médico e vergonha. CONCLUSÕES: O médico é a principal fonte de informação sobre o exame de Papanicolau. Entretanto, mulheres que vão a consultas com maior freqüência, embora apresentem prática mais adequada do exame, possuem baixa adequação de conhecimento e atitude frente ao procedimento, sugerindo que não estejam recebendo as informações adequadas sobre o objetivo do exame, suas vantagens e benefícios para sua saúde.
Seasonal variation of potential flavivirus vectors in an urban biological reserve in northeastern Brazil
(Journal of Medical Entomology, 2009-11) Medeiros, Arlinete S.; Marcondes, Carlos B.; Azevedo, Paulo R. M. de; Jerônimo, Selma M. R.; Silva, Virginia P. Macedo e; Ximenes, Maria de Fátima Freire de Melo
Although yellow fever (YF) has not been reported on the eastern coast of Brazil since 1942, there was a reemergence of dengue fever in Brazil in 1987 due to the reintroduction of Aedes aegypti (L.). To assess areas of potential risk for transmission of vector-borne diseases, a surveillance system was placed in a large Atlantic Forest reserve in Natal, Rio Grande do Norte, Brazil, where in 2004 unexplained epizootics were reported among marmosets. The etiologic agent causing the mortality in marmosets has not been identiÞed. Wyeomyia bourrouli Lutz, Haemagogus leucocelaenus Dyar & Shannon, Ae. aegypti, Aedes albopictus (Skuse), Ochlerotatus scapularis Rondani, Ochlerotatus serratus Theobald, Ochlerotatus taeniorhynchus Wiedemann, Culex quinquefasciatus Say, and Limatus durhami Theobald were collected in the park and in the proximity of the households adjacent to the park. Seasonal abundance ßuctuation was signiÞcant for Ae. aegypti, Ae. albopictus, Ochlerotatus scapularis (Rondani), and Hg. leucocelaenus. Eggs of Ae. aegypti, Ae. albopictus, and Hg. leucocelaenus were more frequently found at the conclusion of the rainy season. A signiÞcant negative correlation between the number of Ae. albopictus collected and temperature was observed (r 0.50), i.e., for each 1 C increase in temperature, the number of specimens collected decreased eight-fold. The Þndings reported herein reinforce the need for a sustainable arboviral surveillance program in this area to decrease the potential risk of emergence of vector borne diseases as YF.
Modelo de regressão Weibull para estudar dados de falha de equipamentos de sub-superfície em poços petrolíferos
(Associação Brasileira de Engenharia de Produção, 2010-03) Dantas, Maria Aldilene; Valença, Dione Maria; Freire, Michell Platiny da Silva; Medeiros, Pledson Guedes de; Silva, Damião Nóbrega da; Aloise, Dario José
Este trabalho apresenta um estudo de confiabilidade em dados relativos ao tempo de vida de poços petrolíferos terrestres da Petrobras, produtores de óleo na Bacia Potiguar (RN/CE). O objetivo do estudo foi, com base em um conjunto de dados sobre ocorrências de falhas, verificar a existência do relacionamento entre o tempo de vida dos poços e algumas de suas características, como método de elevação, nível de produção, BSW (Basic Sediments and Water ), razão gás óleo (RGO), unidade operacional de origem, entre outras. Os dados foram obtidos de um estudo retrospectivo de uma amostra com 450 poços-colunas que se encontravam em funcionamento no período de 2000 a 2006, escolhida de forma a representar todos os poços da bacia RN/CE. Foi realizada uma modelagem probabilística dos dados relativos à primeira falha através do ajuste do modelo de regressão Weibull. O modelo se mostrou apropriado para ajustar os dados e foi possível identificar, através do teste da razão de verossimilhança, quais e de que forma algumas características influenciam o tempo até a falha dos poços.
Prevalence of human papillomavirus in archival samples obtained from patients with cervical pre-malignant and malignant lesions from Northeast Brazil
(BMC Research Notes, 2010-04) Fernandes, José V.; Meissner, Rosely V.; Carvalho, Maria G. F; Fernandes, Thales A. A. M.; Azevedo, Paulo R. M.; Sobrinho, João S.; Prado, José C. M.; Villa, Luisa L.
Background: Human Papillomavirus (HPV) is considered as a necessary, but not sufficient, cause of cervical cancer. In this study, we aimed to assess the prevalence of HPV in a series of pre-malignant and malignant cervical lesion cases, to identify the virus genotypes, and to assess their distribution pattern according to lesion type, age range, and other considered variables. The samples were submitted to histopathological revision examination and analysed by polymerase chain reaction (PCR) for the presence of HPV DNA, followed by HPV typing by dot blot hybridisation. Findings: Of the analysed samples, 53.7% showed pre-malignant cervical lesions, and 46.3% presented with cervical cancer. Most cancer samples (84.1%) were classified as invasive carcinoma. The mean age of these cancer patients was 47.3 years. The overall HPV prevalence was 82.4% in patients with pre-malignant lesions and 92.0% in the cancer patients. HPV 16 was the most prevalent type, followed by HPV 18 and 58, including both single and double infections. Double infection was detected in 11.6% of the samples, and the most common combination was HPV 16+18. Conclusions: Cervical cancer appears to occur in women in a lower age range in the studied area, compared to the situation in other Brazilian regions. Furthermore, among the patients with CIN 3 and those with cancer, we observed a higher proportion of married women, women with more than one sexual partner, smokers, and individuals with less than an elementary education, relative to their counterparts. Findings: The overall HPV prevalence was 82.4% in patients with pre-malignant lesions and 92.0% in the cervical cancer patients from Northeast Brazil. HPV 16 was the most prevalent type, followed by HPV 18 and 58. The most common double infection was HPV 16+18. Cervical cancer appears to occur in women in a lower age range in the Northeast Brazil. Among the patients with CIN 3 and those with cancer, we observed a higher proportion of married women, women with more than one sexual partner, smokers, and individuals with less than an elementary education, relative to their counterparts.
Eficiência, gestão e meio ambiente na carcinicultura do Rio Grande do Norte
(2010-12-07) Silva, Jorge Luiz Mariano da; Sampaio, Luciano Menezes Bezerra
Association between dyslipidemia and anthropometric indicators in adolescents
(Nutrición Hospitalaria, 2011) Lima, S. C. Vieira Cunha; Lyra, C. Oliveira; Pinheiro, L. Galvão Bacurau; Azevedo, P. R. Medeiros de; Arrais, R. F.; Pedrosa, L. F. Campos
The dyslipidemia associated with excess weight is a risk profile global call for cardiovascular disease (CVD). The aim of this study was to investigate the association between dyslipidemias and other risk factors for cardiovascular diseases (CVD) in adolescents, considering sexual maturation. A cross-sectional study was carried out with 432 adolescents from public schools, aged 10-19 years. The correlations between the variables from the lipid profile and the Body Mass Index (BMI), waist circumference (WC), waist-to-height ratio (WHtR), sexual maturation, familial history and maternal education were evaluated using Pearson’s correlation coefficient. Low high-density lipoprotein cholesterol (HDL-C) was the most prevalent dyslipidemia (50.5%), regardless of gender. There were significant correlations between triglycerides and BMI (r = 0.30, p<0.01), WC (r = 0.32, p < 0.01) and WHtR (r = 0.33, p < 0.01). The linear model, which took into consideration sexual maturation, age and BMI, explain about 1 to 10.4% of the lipid profile variation. The low HDL-c was the most prevalent dyslipidemia in all adolescents and hypertriglyceridemia was most prevalent in overweight adolescents. Associations between dyslipidemias and anthropometric indicators (BMI and RCA) detected in this study can generate the hypothesis of the risk factors for CVD in adolescents.
Comparative immunohistochemical expression of RANK, RANKL and OPG in radicular and dentigerous cysts
(Elsevier, 2011-05) Moraes, Maiara de; Lucena, Hévio Freitas de; Azevedo, Paulo Roberto Medeiros de; Queiroz, Lélia Maria Guedes; Costa, Antonio de Lisboa Lopes
Objective: Receptor activator of nuclear factor-kB (RANK), RANK ligand (RANKL) and osteoprotegerin (OPG) are members of the superfamily of ligands and receptors of tumour necrosis factor family involved in bone metabolism. The formation, differentiation and activity of osteoclasts are regulated by these proteins. To clarify the roles of osteoclast regulatory factors in cystic expansion of odontogenic cysts, expression of these proteins were analysed in radicular and dentigerous cysts. Design: The immunohistochemistry expression of these biomarkers were evaluated and measured in lining epithelium and fibrous capsule of the radicular (n = 20) and dentigerous cysts (n = 20). Results: A similar expression in lining epithelium was observed in the lesions. The fibrous capsule of dentigerous cyst showed a higher content of RANK-positive and RANKL-positive cells than fibrous capsule of radicular cyst. In the lining epithelium the RANKL/OPG ratio showed higher numbers of OPG-positive than RANKL-positive cells, whereas fibrous capsule of the cysts had a tendency to present a similar expression (OPG = RANKL). Conclusion: Ours findings indicate the presence of RANK, RANKL and OPG in cysts. Moreover, increased expression of OPG compared to RANKL in the lining epithelium could contribute to the differential bone resorption activity in theses lesions.
Human papillomavirus prevalence in women with normal cytology and with cervical cancer in Natal, Brazil
(Molecular Medicine Reports, 2011-08) Fernandes, José Verissímo; Meissner, Rosely de Vasconcellos; Carvalho, Maria Goretti F.; Fernandes, Thales A. A. de Medeiros; Azevedo, Paulo Roberto Medeiros; Azevedo, Judson Weber Verissímo de; Araújo, Josélio Maria G. de
This study analyzed the prevalence of human papillomavirus (HPV) in cervical specimens obtained from women with normal cytology and with cervical cancer, in order to evaluate their correlation with health status and demographic characteristics, as well as sexual and reproductive activity in women treated at a cancer reference hospital in Natal, Northeast Brazil. A total of 158 women were divided into 2 groups according to their health status: group I comprised 110 women with normal cytology, and group II comprised 48 women with cervical cancer. Cervical smears were analyzed by cytological or histopathological examination for the detection of cytological alterations, and by PCR for HPV DNA detection using MY09/11 primers, followed by HPV genotyping by dot blot hybridization. Results showed overall HPV prevalence to be 24.5% in group I, with 19.1% of patients having single infection and 5.4% double infection. The HPV prevalence in group II was 85.4%, with 79.2% of patients having single and 6.2% double infection. We identified 10 different HPV genotypes, most with high oncogenic potential. HPV 16 was the most prevalent genotype in the two studied groups, followed by HPV 58 and HPV 18. High-risk HPV genital infection, chronological age, ethnicity, early onset of sexual and reproductive activities, multiple sexual partners and smoking increased the risk for cervical cancer.
Frequência e perfil de resistência de Klebsiella spp. em um hospital universitário de Natal/RN durante 10 anos
(Jornal Brasileiro de Patologia e Medicina Laboratorial, 2011-12) Oliveira, Claudio Bruno Silva de; Dantas, Valéria Cristina Ribeiro; Neto, Renato Motta; Azevedo, Paulo Roberto Medeiros de; Melo, Maria Celeste Nunes de
Introdução: As espécies de Klebsiella spp. podem causar vários tipos de infecções, principalmente hospitalares, e têm merecido destaque pelos seus variados e emergentes mecanismos de resistência. Objetivos: Determinar a frequência de isolamento e a caracterização do perfil de resistência de Klebsiella spp. em um hospital universitário durante um período de 10 anos e, ainda, avaliar a tendência para o crescimento dessa resistência. Material e método: Fez-se um estudo descritivo e retrospectivo a partir de dados coletados nos livros de registro do Laboratório de Microbiologia Clínica do hospital investigado, correspondentes ao período de janeiro de 1999 a dezembro de 2008. Resultado: A frequência de isolamento de Klebsiella spp. foi de 13,4% com predominância em uroculturas (56,4%). Houve aumento significativo na resistência para a maioria dos antimicrobianos testados ao longo do período analisado com tendência para o crescimento da mesma. Nesse período, isolou-se 23% de Klebsiella spp. com fenótipo produtor de betalactamases de amplo espectro (ESBL). Discussão: O isolamento de Klebsiella spp. resistente a antimicrobianos em amostras de origem clínica e a detecção da tendência do crescimento da resistência, inclusive às drogas de reserva terapêutica, são motivos de grande preocupação. Nesse hospital, a implantação de métodos de triagem e de confirmatórios para os mecanismos de resistência de Klebsiella spp. poderiam auxiliar no diagnóstico e no tratamento das infecções causadas por esse microrganismo. Conclusão: A tendência de crescimento na resistência aos antibióticos detectada neste estudo reforça a importância de monitoramentos contínuos. Estes elucidam características locais, orientando para melhores medidas de controle.
False-alarm and non-detection probabilities for on-line quality control via HMM
(International Journal of Mathematical Analysis, 2012) Dorea, C.C.Y.; Gonçalves, C.R.; Medeiros, P.G.; Santos, W.B.
On-line quality control during production calls for monitoring produced items according to some prescribed strategy. It is reasonable to assume the existence of system internal non-observable variables so that the carried out monitoring is only partially reliable. In this note, under the setting of a Hidden Markov Model (HMM) and assuming that the evolution of the internal state changes are governed by a two-state Markov chain, we derive estimates for false-alarm and non-detection malfunctioning probabilities. Kernel density methods are used to approximate the stable regime density and the stationary probabilities. As a side result, alternative monitoring strategies are proposed.
On-line process control of the number of non-conformities in the inspected item
(Brazilian Journal of Operations and Production Management, 2012) Vasconcelos, Renata Mendonça Rodrigues; Medeiros, Pledson Guedes de; Ho, Linda Lee
Generally, production systems as automatic welding process, production of ceramic products, making clothes use automatic control and to evaluate the quality of their production processes, they employ on-line process control. The control system consists of a periodic inspection of one item after every m produced items. The number of non-conformities is monitored in the inspected item and if it exceeds the control limit, then it is decided that the process is out-of-control and the process is stopped for adjustment, otherwise the production continues. The process starts leading the system to operate out of control. The process remains in these conditions until the change is detected and the process adjusted. After adjustment, the process returns to operate in-control. The aim of this paper is to present an economic approach to monitor the rate of non-conformities in a production by on-line process control. To design such type of process, an average cost per item produced is achieved through the properties of an ergodic Markov chain and the two required parameters: the inspection interval and the upper control limit are obtained by minimizing the average cost per produced item. A numerical example illustrates the proposal. cost per item: the probability of a shift in the parameter of Poisson distribution; cost to send limit and the cost of adjustment.

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