The likelihood ratio and its graphical representation

Diagnostic tests are important clinical tools. Bayes’ theorem and Bayesian approach are important methods for interpreting test results. The Bayesian factor, the so-called likelihood ratio, has not always been well-understood. In this article, we try to discuss the likelihood ratio and its value for a specific test result, a positive or negative test result, and a range of test results, along with their graphical representations

The apparent prevalence, the true prevalence

Serologic tests are important for conducting seroepidemiologic and prevalence studies. However, the tests used are typically imperfect and produce false-positive and false-negative results. This is why the seropositive rate (apparent prevalence) does not typically reflect the true prevalence of the disease or condition of interest. Herein, we discuss the way the true prevalence could be derived from the apparent prevalence and test sensitivity and specificity. A computer simulation based on the Monte-Carlo algorithm was also used to further examine a situation where the measured test sensitivity and specificity are also uncertain. We then complete our review with a real example. The apparent prevalence observed in many prevalence studies published in medical literature is a biased estimation and cannot be interpreted correctly unless we correct the value

Determining the SARS-CoV-2 serological immunoassay test performance indices based on the test results frequency distribution

Coronavirus disease 2019 (COVID-19) is known to induce robust antibody response in most of the affected individuals. The objective of the study was to determine if we can harvest the test sensitivity and specificity of a commercial serologic immunoassay merely based on the frequency distribution of the SARS-CoV-2 immunoglobulin (Ig) G concentrations measured in a population-based seroprevalence study. The current study was conducted on a subset of a previously published dataset from the canton of Geneva. Data were taken from two non-consecutive weeks (774 samples from May 4-9, and 658 from June 1-6, 2020). Assuming that the frequency distribution of the measured SARS-CoV-2 IgG is binormal (an educated guess), using a non-linear regression, we decomposed the distribution into its two Gaussian components. Based on the obtained regression coefficients, we calculated the prevalence of SARS-CoV-2 infection, the sensitivity and specificity, and the most appropriate cut-off value for the test. The obtained results were compared with those obtained from a validity study and a seroprevalence population-based study. The model could predict more than 90% of the variance observed in the SARS-CoV-2 IgG distribution. The results derived from our model were in good agreement with the results obtained from the seroprevalence and validity studies. Altogether 138 of 1432 people had SARS-CoV-2 IgG ≥ 0.90, the cut-off value which maximized the Youden’s index. This translates into a true prevalence of 7.0% (95% confidence interval 5.4% to 8.6%), which is in keeping with the estimated prevalence of 7.7% derived from our model. Our model can provide the true prevalence. Having an educated guess about the distribution of test results, the test performance indices can be derived with acceptable accuracy merely based on the test results frequency distribution without the need for conducting a validity study and comparing the test results against a gold-standard test

On determining the sensitivity and specificity of a new diagnostic test through comparing its results against a non-gold-standard test

Diagnostic tests are important clinical tools. To assess the sensitivity and specificity of a new test, its results should be compared against a gold standard. However, the gold-standard test is not always available. Herein, I show that we can compare the new test against a well-established diagnostic test (not a gold-standard test, but with known sensitivity and specificity) and compute the sensitivity and specificity of the new test if we would have compared it against the gold-standard test. The technique presented is useful for situations where the gold standard is not readily available

Effects of Back Reflectors and Thin Metal Covers on the Performance of Near-field Thermophotovoltaic Devices

Highly efficient energy harvesting devices that can recover a large amount of waste energy are of significant interest. Thermophotovoltaics (TPVs) are solid-state devices that can convert thermal energy radiated by a heated emitter into electrical power through the process of photovoltaic effect. In a TPV system, heat is converted to electricity by using a high-temperature emitter to radiate photons, which are then absorbed by a photovoltaic (PV) cell, generating electron-hole pairs and producing electrical power. TPV devices have the advantages of producing High efficiency and being lightweight with low maintenance cost and zero pollution. However, these devices have not been commercialized yet as their power output is low. A promising mechanism for increasing the power output and also the efficiency of the TPV cells is by reducing the gap distance between the emitter and the PV cell of the device to a value that is smaller than the dominant wavelength of thermal radiation (~ 10 mm at room temperature). In this case, thermally radiated evanescent electromagnetic waves, which are only substantial at sub-wavelength distances from the emitter, can also be received by PV cells and contribute to power generation. These devices with a sub-wavelength separation gap between the emitting layer and the received are referred to as near-field thermophotovoltaic (NF-TPV) devices. It has been proposed that using a reflector at the back side of the PV cell, introducing a thin metal cover on the PV cell, and creating an air gap between the PV cell and the back reflector can increase the power output and/or the efficiency of the NF-TPV devices. However, it is not known how the performance of the NF-TPV devices changes with using air gap and thin metal layer simultaneously. This thesis examines the effect of the air gap and the presence of the metal cover layer of the NF-TPV devices for a vacuum gap size of 50 nm between the emitter and the PV cell. Also, a new configuration where both the air gap and the metal cover have been used simultaneously, is proposed. Finally, by using genetic algorithms, novel NF-TPV devices with high efficiency and power output are designed. Based on the results, it is seen that utilizing the air gap increases the efficiency of a NF-TV device having a vacuum gap of 50 nm by 10%. However, the power output of the device decreases in the presence of the air gap due to a reduction in the in-band radiation absorption by the PV cell. The air gap is more beneficial at small vacuum gap sizes, such as 10 nm, where both the power output and the efficiency increase with introducing an air gap. It also is shown that, for a NF-TPV device with a vacuum gap size of 50 nm, depositing a very thin metal layer, around 3 nm, on top of the PV cell increases the power output of the NF-TPV nearly by a factor of 4. In this case, efficiency decreases from 30% to 20%. Therefore, placing a thin metal layer on top of the PV cell is practical for waste heat recovery applications where power output is more important than efficiency. The results of this thesis can provide a guide for designing high-power output and large-efficiency NF-TPV devices that can be used for converting heat from any source, including waste heat, to useful electrical power

What’s in a name, anyway?

We have used names, particularly surnames, to identify people who are related. However, this has been done in various ways in different societies. While in many western countries we use a given (first) name and a surname (family name) to identify a person, in Arab countries, there are in fact no surnames; people use their given name followed by their father’s given name. For example, while my name in a western system is “Farrokh Habibzadeh” (Farrokh is my first name and Habibzadeh is my family name), my name in Arabic documents, say in a visa, would be “Farrokh Naser” (Naser is my father’s first name). Chinese use another style. They usually use their surname followed by their given name. Then, my name in a Chinese system would be “Habibzadeh Farrokh

The influence of safety measures on workers' safety perception and behavior

Memória Descritiva do Projeto de Investigação final de Mestrado apresentado à Faculdade de Letras da Universidade de Coimbra.A valorização do património e do passado de Coimbra tem conhecido uma preocupação crescente ao longo das últimas décadas, culminando com a classificação da Universidade, Alta e Sofia enquanto Património Mundial da UNESCO. O objetivo assumido para a elaboração do presente projeto final, foi, portanto, acompanhar a revalorização de um dos edifícios a ser alvo de recuperação (o Colégio da Santíssima Trindade) e perceber porque é ele importante para a história da cidade.The valorisation of the heritage and history of Coimbra has seen a growing concern over the past decades, culminating in the classification of the University of Coimbra, Alta and Sofia as UNESCO World Heritage site. The aim assumed for the preparation of this final project was to follow the restoration and rehabilitation of one of the buildings of UNESCO’s buffer zone (the Holy Trinity College) and understand why this college is so important for the city’s history