Amostragem de aceitação para variáveis não gaussianas

Tese de doutoramento, Estatística e Investigação Operacional (Probabilidades e Estatística), Universidade de Lisboa, Faculdade de Ciências, 2012No Controlo de Qualidade de um processo de produção, pode proceder-se ao controlo do processo propriamente dito (Controlo Estatístico do Processo), ou ao controlo nas suas fronteiras – Amostragem de Aceitação, por atributos ou por variáveis e ao Planeamento de Experiências. Abordar-se-á, neste trabalho, a Amostragem de Aceitação por variáveis. A Amostragem de Aceitação é utilizada para inspeccionar quer o “input” – matériaprima – quer o “output” – produto final – do processo de produção. A Amostragem de Aceitação determina um procedimento, que se aplicado a uma série de lotes, dá o risco de aceitar lotes com uma determinada qualidade. Por outras palavras, a Amostragem de Aceitação permite assegurar a qualidade e não estimá-la. Um plano de Amostragem de Aceitação apenas aceita ou rejeita lotes, considerando a informação fornecida pela amostra. O caso clássico de Amostragem de Aceitação por variáveis para a percentagem nãoconforme, tratado em normas clássicas [ANSI/ASQC Z1.9, 2008], baseia-se na hipótese de que a variável em estudo segue uma distribuição Gaussiana. Ora, assumir que a característica de qualidade em estudo é Gaussiana, pode em algumas situações ser abusiva, tal como no caso de variáveis assimétricas e/ou com caudas pesadas, dando origem a decisões erradas. A Amostragem de Aceitação para variáveis não Gaussianas, é então, relevante. Quando se está a trabalhar com uma distribuição não Gaussiana, podem construir-se planos específicos de Amostragem de Aceitação, para a distribuição subjacente, com um único limite de especificação (inferior ou superior) e planos com limites de especificação simultâneos, ambos estudados neste trabalho. Na literatura encontram-se alguns estudos limites simultâneos e um único limite de especificação, mas a maioria sobre o caso clássico. Neste trabalho, focar-se-á o problema de determinar planos de amostragem de aceitação para variáveis, com um limite de especificação e limites de especificação simultâneos, com distribuição Exponencial, Gama, Weibull, Fréchet e Gumbel, comparando-se os resultados com os obtidos para o caso Gaussiano (dando maior enfase ao caso em que os parâmetros são desconhecidos, recorrendo-se, neste caso, a métodos de simulação). Se a distribuição real dos dados é bastante assimétrica e/ou possui caudas pesadas, mas é fácil modelar os dados e estimar os seus parâmetros, o que usualmente não acontece, então podem construir-se planos de amostragem específicos. Alternativamente, pode proceder-se à transformação dos dados originais em valores normais, através de uma transformação tipo Box-Cox, que não requer a modelação prévia dos dados, e construir de seguida um plano de Amostragem de Aceitação para o caso clássico – o Gaussiano. ii Considerando as distribuições já referidas anteriormente, os dois métodos são comparados.In the quality control of a production process (of goods and services), from a statistical point of view, focus is either on the process itself with application of Statistical Process Control, or on its frontiers, with application of Acceptance Sampling (AS) – studied here – and Experimental Design. AS is used to inspect either the output process – final product – or the input – initial product. The purpose of AS is to determine a course of action, not to estimate lot quality. AS prescribes a procedure that, if applied to a series of lots, will give a specified risk of accepting lots of given quality. In other words, AS yields quality assurance. An AS plan merely accepts and rejects lots, considering sampling information. The AS by variables is based on the hypothesis that the observed quality characteristics follow a known distribution, namely the Gaussian distribution (classical case of the AS by variables – treated in classical standards, [ANSI/ASQC Z1.9, 2008]). This is sometimes, however, an abusive assumption, as in the case of asymmetric variables and/or heavy tails, that leads to wrong decisions. AS for non-Gaussian, is thus relevant. When we have a non-Gaussian distribution we can build specific AS plans associated with that distribution. In acceptance sampling we can build plans with a single specification limit (upper or lower) or simultaneous specification limits – both situations are studied in this work. In the literature there are few studies on simultaneous limits, single limits on AS, but most of the classical case. In this work we will address the problem of determining acceptance sampling plans by variables with single and simultaneous specification limits for Exponential, Gamma, Weibull, Fréchet and Gumbel distributions, the results being compared to the Gaussian case (greater emphasis will be given to the case of unknown parameters, using, in this case, simulation methods). If the real distribution of data is very asymmetric and/or has heavy tails, but we are able to adequately model the data and estimate its parameters, which usually is not easy, we can use those specific AS plans. Alternatively, we can make the transformation of the original data into normal values through a transformation of the Box-Cox type, which requires no prior modeling process of the data and then use AS plans for the classical case – the Gaussian case. Considering the previous distributions, both methods being compared.Fundação para a Ciência e a Tecnologia (FCT, bolsas PROTEC

Factors influencing amblyopia rehabilitation in occlusive therapy

Amblyopia develops in an early period and is a decrease of visual acuity (unilateral or bilateral) caused by a deprivation of vision or abnormal binocular interaction. Prognosis of Amblyopia is better when occlusive treatment is implemented in an early stage. Visual acuity of amblyopic eye does not improve without effective occlusive therapy. The aim of this study is to identify potential risk factors of noncompliance with treatment when it is implemented by parents in amblyopic children

Parental predictors of poor visual outcome with occlusion treatment for unilateral amblyopia

Aim: Visual acuity outcome of amblyopia treatment depends on the compliance. This study aimed to determine parental predictors of poor visual outcome with occlusion treatment in unilateral amblyopia and identify the relationship between occlusion recommendations and the patient's actual dose of occlusion reported by the parents. Methods: This study comprised three phases: refractive adaptation for a period of 18 weeks after spectacle correction; occlusion of 3 to 6 hours per day during a period of 6 months; questionnaire administration and completion by parents. Visual acuity as assessed using the Sheridan-Gardiner singles or Snellen acuity chart was used as a measure of visual outcome. Correlation analysis was used to describe the strength and direction of two variables: prescribed occlusion reported by the doctor and actual dose reported by parents. A logistic binary model was adjusted using the following variables: severity, vulnerability, self-efficacy, behaviour intentions, perceived efficacy and treatment barriers, parents' and childrens' age, and parents' level of education. Results: The study included 100 parents (mean age 38.9 years, SD approx 9.2) of 100 children (mean age 6.3 years, SD approx 2.4) with amblyopia. Twenty-eight percent of children had no improvement in visual acuity. The results showed a positive mild correlation (kappa = 0.54) between the prescribed occlusion and actual dose reported by parents. Three predictors for poor visual outcome with occlusion were identified: parents' level of education (OR = 9.28; 95%CI 1.32-65.41); treatment barriers (OR = 2.75; 95%CI 1.22-6.20); interaction between severity and vulnerability (OR = 3.64; 95%CI 1.21-10.93). Severity (OR = 0.07; 95%CI 0.00-0.72) and vulnerability (OR = 0.06; 95%CI 0.05-0.74) when considered in isolation were identified as protective factors. Conclusions: Parents frequently do not use the correct dosage of occlusion as recommended. Parents' educational level and awareness of treatment barriers were predictors of poor visual outcome. Lower levels of education represented a 9-times higher risk of having a poor visual outcome with occlusion treatment

Diagnostic performance of visual screening tests in the elderly

The aging of Portuguese population is characterized by an increase of individuals aged older than 65 years. Preventable visual loss in older persons is an important public health problem. Tests used for vision screening should have a high degree of diagnostic validity confirmed by means of clinical trials. The primary aim of a screening program is the early detection of visual diseases. Between 20% and 50% of older people in the UK have undetected reduced vision and in most cases is correctable. Elderly patients do not receive a systematic eye examination unless a problem arises with their glasses or suspicion vision loss. This study aimed to determine and evaluate the diagnostic accuracy of visual screening tests for detecting vision loss in elderly. Furthermore, it pretends to define the ability to find the subjects affected with vision loss as positive and the subjects not affected with the same disease as negative. The ideal vision screening method should have high sensitivity and specificity for early detection of risk factors. It should be also low cost and easy to implement in all geographic and socioeconomic regions. Sensitivity is the ability of an examination to identify the presence of a given disease and specificity is the ability of the examination to identify the absence of a given disease. It was not an aim of this study to detect abnormalities that affect visual acuity. The aim of this study was to find out what´s the best test for the identification of any vision loss

Contributo para a validação da escala de avaliação de competências dos ortoptistas no rastreio visual infantil

Neste estudo de investigação foi analisada a estrutura factorial do questionário de “Competências dos Ortoptistas no Rastreio Visual Infantil - CORVI” e construída uma escala de avaliação de competências no rastreio visual infantil. Questionário CORVI é um questionário específico de autopreenchimento constituído por 47 itens, apresentados sob a forma de afirmações, agrupados por 3 dimensões analíticas de análise adaptadas do Tuning Educacional Structures in Europe (2004): competências sistémicas, competências interpessoais e competências instrumentais. Os itens que o constituem permitem identificar a percepção dos inquiridos sobre as competências que consideram possuir para a prática profissional do rastreio visual infantil. Este questionário foi submetido a validação facial e a pré-teste. A consistência interna foi analisada com recurso à Reliability Analysis com análise do índice alpha de Cronbach (α). Obtiveram-se bons valores de consistência interna para todas as dimensões de análise “Competências Sistémicas” (α = 0,916), “Competências Interpessoais” (α = 0,949) e “Competências Instrumentais” (α = 0,892)

Risk factors for non compliance using occlusive therapy: a case study

Purpose: compliance with treatment is a common problem when treating amblyopic patients. Visual acuity of amblyopic eye does not improve without effective occlusive therapy. The aim of this study is to identify potential risk factors of non-compliance with treatment when it is implemented by family in amblyopic children. Setting: a quantitative transversal study was performed in a public hospital and in a private clinic in Lisbon

Ventilação não invasiva durante o exercício em pacientes com DPOC: revisão sistemática com meta-análise

A DPOC é uma das principais causas mundiais de morbilidade e mortalidade, representando um problema de saúde pública devido ao elevado consumo de recursos sanitários e económicos associados. A reabilitação pulmonar é uma recomendação standard nos cuidados a estes pacientes, de forma a controlar os sintomas da doença e optimizar a capacidade funcional destes indivíduos, reduzindo assim os custos de saúde associados às exacerbações e limitação da actividade e participação. Contudo, em pacientes com DPOC severa o exercício físico pode ser de difícil desempenho devido a dispneia extrema, diminuição da força muscular e fadiga, ou inclusive hipoxemia e dispneia durante pequenos esforços e actividades diárias, limitando a sua qualidade de vida. Assim, a VNI tem sido usada em combinação com o exercício, de forma a melhorar a capacidade de exercício nestes doentes, ainda que sem consenso para a sua recomendação. Assim, tivemos como objectivo verificar se a utilização de VNI durante o exercício é mais eficaz que exercício sem VNI, na dispneia, distância percorrida, gasimetria e estado de saúde, em pacientes com DPOC, através de revisão sistemática e meta-análise

Non invasive ventilation during exercise in COPD patients: a systematic review with meta-analysis

COPD is a major cause of morbidity and mortality worldwide, representing a major public health problem due to the high health and economic resource consumption. Pulmonary rehabilitation is a standard care recommendation for these patients, in order to control the symptoms and optimize the functional capacity, reducing health care costs associated with exacerbations and activity limitations and participation. However, in patients with severe COPD exercise performance can be difficult, due to extreme dyspnea, decreased muscle strength and fatigue. In addition, hypoxemia and dyspnea during efforts and daily activities may occur, limiting their quality of life. Thus, NIV have been used as adjunct to exercise, in order to improve exercise capacity in these patients. However, there is no consensus for this technique recommendation. Our objective was to verify whether the use of NIV during exercise is effective than exercise without NIV in dyspnea, walked distance, blood gases and health status in COPD patients, through a systematic review and meta-analysis

Visual Screening Competencies Questionnaire (VSCQ) and orthoptic practice

The identification of core competencies which are important for undertaking accurate visual screening by orthoptists is considered in this study. The aim was to construct and validate a questionnaire for orthoptists to assess visual screening competency. This study comprised three steps. The first step involved a 69-item self-assessment questionnaire constructed to assess orthoptists' perception of their competencies in visual screening programs for children. This questionnaire was constructed with statements from the Orthoptic Benchmark Statement for Health Care Programmes (Quality Assurance Agency for Higher Education, UK) and included three competency dimensions: interpersonal (IP), instrumental (IT) and systemic (ST). The second step involved questionnaire translation

Occupational stress and coping resources in physiotherapists: a survey of physiotherapists in three general hospitals

Objectives - To identify occupational stressors and coping resources in a group of physiotherapists, and to analyse interactions between subjective levels of stress, efficacy in stress resolution and coping resources used by these professionals. Design - A sample of 55 physiotherapists working in three general hospitals in Portugal completed the Coping Resources Inventory for Stress, the Occupational Stressors Inventory and two subjective scales for stress and stress resolution. Main results - Most physiotherapists perceived that they were moderately stressed (19/55, 35%) or stressed (20/55, 36%) due to work, and reported that their efficacy in stress resolution was moderate (25/54, 46%) or efficient (23/54, 42%). Issues related to lack of professional autonomy, lack of organisation in the hierarchical command chain, lack of professional and social recognition, disorganisation in task distribution and interpersonal conflicts with superiors were identified as the main sources of stress. The most frequently used coping resources were social support, stress monitoring, physical health and structuring. Perceived efficacy in stress resolution was inversely related to perceived level of occupational stress (r = −0.61, P < 0.01). Significant correlations were found between several coping resources and the perceived level of stress and efficacy in stress resolution. Associations between problem solving, cognitive restructuring and stress monitoring and both low levels of perceived stress and high levels of perceived efficacy were particularly strong. Implications for practice - The importance of identifying stressors and coping resources related to physiotherapists’ occupational stress, and the need for the development of specific training programmes to cope with stress are supported