The aim of this thesis was to investigate the use of effect sizes to report the results of statistical credit rating models in a more practical way. Rating systems in the form of statistical probability models like logistic regression models are used to forecast the behaviour of clients and guide business in rating clients as “high” or “low” risk borrowers. Therefore, model results were reported in terms of statistical significance as well as business language (practical significance), which business experts can understand and interpret. In this thesis, statistical results were expressed as effect sizes like Cohen‟s d that puts the results into standardised and measurable units, which can be reported practically. These effect sizes indicated strength of correlations between variables, contribution of variables to the odds of defaulting, the overall goodness-of-fit of the models and the models‟ discriminating ability between high and low risk customers. / Statistics / M. Sc. (Statistics)

Abstract Purpose: The United States has the highest rate of teen pregnancy of any industrialized nation. Adolescents who have their first sexual intercourse at a young age are at increased risk for teen pregnancies and acquiring a sexually transmitted disease. This study examines predictors of early onset sexual intercourse in male and female residents of the United States. Methods: A nationally representative sample of N=7,643 females and N=4.928 males ages 15-44 was procured from the 2002 National Survey of Family Growth (NSFG), Cycle 6. Age at first sexual intercourse was used to define early onset of sexual debut(<18 years). Socio-demographic and behavioral characteristics of the respondents, demographic and selected reproductive characteristics of the respondent's parents were examined using multiple logistic regression modeling. Results: Non-fispanic black, being raised without both parents, having a mother less than 18 years old at the age of first birth and age difference between partners were significant predictors of early onset of sexual intercourse for both males and females. Maternal education less than high school was a significant protective factor for female respondents [OR=0.72 (95%CI=0.58- 0.90)] and paternal education completed high school only [OR=1.4 (95% CI=l. 1-1.7)] was a significant risk factor for male respondents. Conclusions: Racelethnicity, age difference between partners, not being raised by both parents, having a mother who had her first birth before the age of 18 and parental education are important predictor variables. Further study should be conducted to investigate the protective effect of lack of maternal education for female respondents. Intervention programs for teen pregnancy and sexually transmitted infection prevention should target these at risk groups.

Um dos estudos observacionais mais difundidos e usados em epidemiologia veterinária é o estudo do tipo transversal. Sua popularidade ocorre por fatores como baixo custo e rapidez comparados com outros tipos de estudos, além de ajudar a estimar a prevalência de uma doença (desfecho) e postular fatores associados com o desfecho, que poderão ser confirmados como fatores causais em outros tipos de estudos epidemiológicos. Porém, este tipo de estudo apresenta dois importantes desafios: a dependência dos dados, muito frequente dada a típica estrutura populacional de animais dentro do mesmo rebanho ou fazenda e a escolha da medida de associação para desfechos binários, tão frequentes neste modelo de estudo. Com o objetivo de contribuir com a compreensão global da epidemiologia do aborto bovino associado à Neospora caninum tendo em conta a estrutura populacional, construiu-se um modelo misto com os dados de um estudo transversal realizado em duas regiões do Rio Grande do Sul. Usaram-se dados de 60 propriedades amostradas em duas regiões (noroeste e sudeste) e 1256 bovinos. A percentagem de aborto dentro de cada rebanho variou entre 1% e 30%. Vacas soropositivas tiveram 6,63 vezes mais chances de ter histórico de aborto (IC 95%: 4,41-13,20). As chances de uma vaca ter histórico de aborto foram 5,18 vezes maiores na região noroeste em relação à região sudeste (IC 95%: 1,83-20,80). Um coeficiente de correlação intraclasse de 16% foi estimado, indicando que 16% da variação da ocorrência de abortamentos não explicados pelos efeitos fixos foram devido as fazendas. Já na segunda parte deste trabalho, uma revisão sistemática foi realizada considerando um conjunto diverso de revistas e jornais com o objetivo de verificar os métodos estatísticos usados e a adequação das interpretações das medidas de associação estimadas em estudos transversais na área de medicina veterinária. Um total de 62 artigos foi avaliado. A revisão mostrou que, independentemente do nível de prevalência relatado no artigo, 96% deles empregou regressão logística e, portanto, estimaram razão de chances (RC). Nos artigos com prevalência superior a 10%, 23 deles fizeram uma interpretação adequada da RC como uma “razão de chances” ou simplesmente não fizeram uma interpretação direta da RC, enquanto 23 artigos interpretaram de forma inadequada a RC, considerando-a como risco ou probabilidade. Entre os artigos com prevalência inferior a 10%, apenas três interpretaram a RC como uma “razão de chances”, cinco interpretaram como risco ou probabilidade e em um, apesar de ter estimado a razão de prevalências (RP), foi interpretado de forma inadequada. Paralelamente, com o objetivo de exemplificar o uso de métodos estatísticos que estimam diretamente a razão de prevalências (RP), medida mais adequada para os estudos transversais, um conjunto de dados obtidos a partir de um estudo transversal sobre a ocorrência de anticorpos (AC) contra o vírus da diarreia viral bovina (BVDV) foi usado. Os AC foram medidos em amostras de tanque de leite de rebanhos leiteiros localizados no estado do Rio Grande do Sul, em que os possíveis fatores associados puderam ser avaliados. Entre os métodos utilizados, as maiores discrepâncias nas medidas de associação estimadas foram observadas com a regressão logística tomando-se como referência a regressão log-binomial. Finalmente, é importante que este tipo de desafio seja atendido pelos pesquisadores que realizam estudos transversais, ou seja, considerar a estrutura das populações nas análises, cuidado ao escolher o tipo de modelo estatístico empregado para desfecho binário e interpretação dos estimadores. / The commonest study design used in veterinary epidemiology is the cross-sectional study. Its popularity lies on the fact of the short time needed and low costs compared with other types of studies; moreover, this type of study estimates prevalence and associated factors, which may be elucidated as causal in another type of epidemiological studies. However, this type of study presents two major challenges: a very common dependence between data given the typical structure of the animal population, i.e., animals within herds or farms and the choice of measure of association for binary outcomes, frequently used in this type of study. In order to contribute to the understanding of the epidemiology of bovine abortion associated with Neospora caninum, a mixed model accounting for the hierarchical structure of cattle population using data from a cross-sectional study conducted in two regions (northwest and southeast) of Rio Grande do Sul was made. Data from 60 dairy herds and 1256 sampled cattle were used. The percentage of abortions in each herd ranged between 1% and 30%. Seropositive cows were 6.63 times more likely to have a history of abortion (95% CI: 4.41 to 13.20). The chances of a cow have a history of abortion were 5.18 times higher in the northwest comparing with the southeast region (95% CI: 1.83 to 20.80). An intraclass correlation coefficient (ICC) of 16% was estimated which means that 16% of the variation in abortion occurrence not explained by the fixed effects is due to farms. In the second part of this work, a systematic review was conducted considering a range of journals and newspapers in order to verify the statistical methods used and the adequacy of the interpretations of the measures of association estimated in cross-sectional studies from the veterinary medicine field. A total of 62 articles were revised. The review showed that, regardless of the reported prevalence, 96% of them employed logistic regression, therefore estimating odds ratio (OR). From the articles that reported prevalence rates above 10%, 23 of them did a proper interpretation of OR as an odds ratio, or simply did not make a direct interpretation of the OR, while 23 articles interpreted improperly the OR as a risk or probability. Among the articles that reported prevalence rates lower than 10%, only three interpreted the OR as an odds ratio, five interpreted as a risk or probability and only one, despite the estimated prevalence ratio (PR), it was improperly interpreted. Meanwhile, in order to exemplify the use of statistical methods to estimate directly the PR, the more appropriate measure of association in cross-sectional studies, a data set obtained from a cross-sectional study to estimate the occurrence of antibodies (AB) against bovine viral diarrhea virus (BVDV) in milk was used; AB were measured in bulk tank samples from dairy herds located in the state of Rio Grande do Sul, Brazil, and also possible associated factors were estimated. Among the methods used, major discrepancies in the measures of association estimated were observed with the logistic regression, comparing with the log-binomial regression. Finally, it is important that such challenges are met by the researchers that undertake cross-sectional studies.

The aim of this thesis was to investigate the use of effect sizes to report the results of statistical credit rating models in a more practical way. Rating systems in the form of statistical probability models like logistic regression models are used to forecast the behaviour of clients and guide business in rating clients as “high” or “low” risk borrowers. Therefore, model results were reported in terms of statistical significance as well as business language (practical significance), which business experts can understand and interpret. In this thesis, statistical results were expressed as effect sizes like Cohen‟s d that puts the results into standardised and measurable units, which can be reported practically. These effect sizes indicated strength of correlations between variables, contribution of variables to the odds of defaulting, the overall goodness-of-fit of the models and the models‟ discriminating ability between high and low risk customers. / Statistics / M. Sc. (Statistics)

人民幣是否可能成為另一個重要的國際貨幣，甚至挑戰美元的國際地位？此即本論文的問題意識。對此，本論文進一步提出三個研究問題：一是如何測量當前的人民幣國際化程度？二是如何測量當前的人民幣資本開放程度？三是資本開放對於人民幣國際化程度的影響為何？ 為此，本研究利用主成分分析（PCA），以建構人民幣國際化程度（CIDI）與人民幣資本帳開放程度（CAOI）。其次再利用動態追蹤資料模型──系統一般動差估計法（SGMM），以檢證各項人民幣綜合競爭力對於貨幣國際化程度的影響。最後，本研究進一步梳理人民幣資本帳開放的進程，並結合上述所有實證分析的結果，進而預估漸進資本開放下人民幣國際化的前景。研究對象包括人民幣在內的33種國際貨幣，研究時間則起自1999年歐元成立，迄於2009年。 本論文的發現三：一是，當前人民幣國際化程度進展相當快速。但截至2009年年底，人民幣國際化程度還很低，遠落後於美元、歐元、日圓，以及英鎊等主要國際貨幣。不僅如此，人民幣國際化程度也遜於俄羅斯盧布、巴西里拉，以及印度盧比等開發中國家所發行的貨幣。 二是，過去10年來，人民幣資本帳開放程度不升反降，截至2009年年底，人民幣的資本帳開放程度維持在零，這表示：人民幣是世界上管制最為嚴格的貨幣。相對而言，美元、歐元、日圓，以及英鎊的資本帳開放程度至少都在70％以上，特別是英鎊的資本帳開放程度更趨近於完全開放。 三是，根據SGMM的實證結果顯示，網路外部性、經濟規模、金融市場規模、貨幣穩定度，以及資本開放程度都是影響貨幣國際化程度的關鍵因素。在此基礎上，本研究利用發生機率（odds ratio），以計算不同資本開放情境下，人民幣成為前10大國際貨幣的可能性。結果顯示，如果人民幣的資本帳開放到73％左右，人民幣便可擠進前10大國際貨幣（發生機率為65.6％）。 不過，這只是最為保守的估計。原因有二：一是，隨者中國經濟實力的崛起，以及人民幣預期升值的脈絡下，國際市場對於人民幣的需求原本就很高。此時，人民幣資本帳如果能適時開放，則人民幣的國際持有將大幅增加。換言之，本研究沒有考量到，各貨幣競爭力因素與資本開放程度之間的加乘效果。 二是，資本開放不僅直接對貨幣國際化程度產生影響，也會透過擴大金融市場規模與網路外部性等其他貨幣競爭力因素，間接對貨幣國際化程度造成影響。這間接效果，本研究也沒有考量到。因此，可以預期的是，只要人民幣資本帳能夠漸進開放，人民幣國際化的前景將比本研究所預估的高出許多。 / This paper discusses whether the Renminbi (RMB) will become an international currency, even challenging to the U.S. dollar. In order to examine above question, this paper take the following three steps: 1. By using principal component analyses (PCA), this paper constructs two indices: currency internationalization degree index (CIDI) and capital account liberalization degree index (CAOI); 2. By using dynamic panel data model-system generalized method of moment (SGMM), this paper analyzes factors affect the CIDI, including economic and trade size, financial system, network externalities, confidence in the currency’s value, and CAOI; 3. According to the PCA and SGMM results, this paper calculates the odds ratio of RMB becoming important international currency. The reserch achieved the following results. First, the degree of internationalization of the RMB progress very fast, but the RMB CIDI is still very low, its CIDI far behinds the dollar, euro, Japanese yen, and pounds. Second, over the past 10 years, RMB CAOI is not increased but decreased. Its CAOI is at zero in 2009, this means that: the RMB is the most stringent controls in the world currency. In contrast, U.S. dollars, euros, yen, and pound CAOI are at least in more than 70%. Third, according to the SGMM results, economic size, financial system, network externalities, confidence in the currency’s value, and CAOI are key factors affect the CIDI. Based on this output, this paper forecasted that if the RMB CAOI is open to about 73%, RMB could be squeezed into the top 10 of the international currency. (The odds ratio is 65.6%) It is noteworthy that this is only the lowest estimates. This is because that this paper did not consider the interaction effects of each currency competitiveness factors and CAOI. Therefore, if RMB CAOI continues open, the prospect of RMB CIDI is much higher than estimated by this paper.

Bakgrund: Tidigare studier har identifierat flera riskfaktorer, såsom kost, fysisk aktivitet och BMI, gällande kolorektal cancer. Att analysera kost utifrån kostmönster istället för enskilda livsmedel har visat sig vara effektivt för att undersöka risker för kolorektal cancer. Datamaterial samlades in med hjälp av en fall-kontroll studie med 257 fall och 805 kontroller. Syfte: Identifiera faktorer som ger en höjd eller sänkt risk för kolorektal cancer utifrån områdena kost, fysisk aktivitet och BMI. Metod: Faktoranalys användes för att upptäcka kostmönster. Logistisk regression användes för att skatta oddskvoter och 95 % konfidensintervall. Resultat: Tio stycken kostmönster erhölls från faktoranalysen. Kostmönstren ”Läsk, juice och mjölkprodukter” (OR=1,288; ORQ4=2,159), ”Te, men inte kaffe”(OR=1,228; ORQ3=1,891; ORQ4=1,668) och ”Fågel, rött kött och fisk”( ORQ4=1,724) gav alla en ökad risk. Däremot visade kostmönstret ”Mat från säd och ost”( ORQ2=0,546; ORQ4=0,592) en minskad risk. BMI för tio år sedan (OR=1,079; ORÖvervikt=1,491; ORFetma=2,260) identifierades som en riskfaktor. Att arbeta inom stillasittande (OR=0,975; OR&gt;15 år=0,517) och mellanaktiva (OR=0,977; OR6-10 år=0,497;OR&gt;15 år=0,565) yrken visade på en minskad risk. Slutsats: Flera kostmönster visade sig vara riskfaktorer, detta gäller även BMI för tio år sedan. Kostmönstret ”Mat från säd och ost” och att arbeta i fysiskt lätta till medeltunga yrken visade sig vara skyddande faktorer. / Background: Previous studies have shown several risk factors for developing colorectal cancer such as diet, physical activity and BMI. The method of analyzing diets based on dietary patterns, rather than individual food items, have been shown to be effective when investigating the colorectal cancer risk. The data was collected using a case-control study of 257 cases and 805 controls. Aim: Identify factors that cause increased or decreased risk in developing colorectal cancer based on diet, physical activity and BMI. Methods: Factor analysis was conducted to identify dietary patterns. Logistic regression was used to estimate odds ratio and 95 % confidence interval. Results: Factor analysis conducted ten dietary patterns, three of these patterns showed an increased risk “Soft drinks, juice and milk products” (OR=1,288; ORQ4=2,159), “Tea, but not coffee” (OR=1,228; ORQ3=1,891; ORQ4=1,668) and “Poultry, red meats and fish” (ORQ4=1,724).The dietary pattern “Food based on grain and cheese” (ORQ2=0,546; ORQ4=0,592) showed a decreased risk. BMI ten years ago (OR=1,079; OROverweight=1,491; ORObese=2,260) identified as a risk factor. To work in sedentary (OR=0,975; OR&gt;15 years=0,517) or physically medium heavy (OR=0,977; OR6-10 years=0,497; OR&gt;15 years=0,565) occupations indicated a decreased risk. Conclusions: Several dietary patterns has been identified as risk factors, this also includes BMI ten years ago. The dietary pattern “Food based on grain and cheese” and to work in sedentary or physically medium heavy occupations proved to be protective factors.

給定聯合分配，可以容易地導出對應的條件分配。反之，給定條件分配的資訊，是否能導出對應的聯合分配呢？例如根據O. Paul et al.（1963，1968）對造成心血管疾病因素之追蹤研究，可得出咖啡量、吸菸量及是否有心血管疾病三者間的條件機率模型資料，是否能找到對應的聯合機率模型，以便可以更深入地研究三者之關係，是一個重要的議題。在選定參考點下，Chen（2010）提出以勝算比法找條件密度函數相容的充要條件，以及在相容性成立時，如何求得聯合分配。在二維中，當兩正值條件機率矩陣不相容時，郭俊佑（2013）以幾何平均法修正勝算比矩陣，並導出近似聯合分配，同時利用幾何平均法之特性，提出最佳參考點之選擇法則。本研究以二維的勝算比法為基礎，探討三維離散的相容性問題，獲得下列幾項結果：一、證明了三個三維條件機率矩陣相容的充要條件就是兩兩相容。二、當三維條件機率矩陣不相容時，利用幾何平均法導出近似聯合分配。三、利用兩兩相容的充要條件，導出三維條件機率矩陣相容的充要條件，並證明該充要條件與Chen的結果一致。四、在幾何平均法下，提出最少點法，有效率地找出最佳參考點，以產生總誤差最小的近似聯合分配。五、設計出程式檢驗三維條件機率矩陣是否相容，並找出最佳參考點，同時比較最少點法與窮舉法之間效率的差異。 / Given a joint distribution, we can easily derive the corresponding fully conditional distributions. Conversely, given fully conditional distributions, can we find out the corresponding joint distribution? For example, according to a longitudinal study of coronary heart disease risk factors by O. Paul et al. (1963, 1968), we obtain conditional probability model data among coffee intake, the number of cigarettes smoked and whether he/she has coronary heart disease or not. Whether we can find out the corresponding joint distribution is an important issue as the joint distribution may be used to do further analyses. Chen (2010) used odds ratio method to find a necessary and sufficient condition for their compatibility and also gave the corresponding joint distribution for compatible situations. When two positive discrete conditional distributions in two dimensions are incompatible, Kuo (2013) used a geometric mean method to modify odds ratio matrices and derived an approximate joint distribution. Kuo also provided a rule to find the best reference point when the geometric mean method is used. In this research, based on odds ratio method in two dimensions, we discuss their compatibility problems and obtain the following results on three-dimensional discrete cases. Firstly, we prove that a necessary and sufficient condition for the compatibility of three conditional probability matrices in three dimensions is pairwise compatible. Secondly, we extend Kuo’s method on two-dimensional cases to derive three-dimensional approximate joint distributions for incompatible situations. Thirdly, we derive a necessary and sufficient condition for the compatibility of three conditional probability matrices in three dimensions in terms of pairwise compatibility and also prove that this condition is consistent with Chen’s results. Fourthly, we provide a minimum-points method to efficiently find the best reference point and yield an approximate joint distribution such that total error is the smallest. Fifthly, we design a computer program to run three-dimensional discrete conditional probability matrices problems for compatibility and also compare the efficiency between minimum-points method and exhausting method.

Research Doctorate - Doctor of Philosophy (PhD) / Interval estimation of the Binomial parameter è, representing the true probability of a success, is a problem of long standing in statistical inference. The landmark work is by Bayes (1763) who applied the uniform prior to derive the Beta posterior that is the normalised Binomial likelihood function. It is not well known that Bayes favoured this ‘noninformative’ prior as a result of considering the observable random variable x as opposed to the unknown parameter è, which is an important difference. In this thesis we develop additional arguments in favour of the uniform prior for estimation of è. We start by describing the frequentist and Bayesian approaches to interval estimation. It is well known that for common continuous models, while different in interpretation, frequentist and Bayesian intervals are often identical, which is directly related to the existence of a pivotal quantity. The Binomial model, and its Poisson sister also, lack a pivotal quantity, despite having sufficient statistics. Lack of a pivotal quantity is the reason why there is no consensus on one particular estimation method, more so than its discreteness: frequentist (unconditional) coverage depends on è. Exact methods guarantee minimum coverage to be at least equal to nominal and approximate methods aim for mean coverage to be close to nominal. We agree with what seems like the majority of frequentists, that exact methods are too conservative in practice, and show additional undesirable properties. This includes more recent ‘short’ exact intervals. We argue that Bayesian intervals based on noninformative priors are preferable to the family of frequentist approximate intervals, some of which are wider than exact intervals for particular data values. A particular property of the interval based on the uniform prior is that its mean coverage is exactly equal to nominal. However, once committed to the Bayesian approach there is no denying that the current preferred choice, by ‘objective’ Bayesians, is the U-shaped Jeffreys prior which results from various methods aimed at finding noninformative priors. The most successful such method seems to be reference analysis which has led to sensible priors in previously unsolved problems, concerning multiparameter models that include ‘nuisance’ parameters. However, we argue that there is a class of models for which the Jeffreys/reference prior may be suboptimal and that in the case of the Binomial distribution the requirement of a uniform prior predictive distribution leads to a more reasonable ‘consensus’ prior.

Homeownership is widely advocated and believed to contribute towards economic activity, employment, wealth creation, economic, political, and neighbourhood stability and financial independence. Despite government’s interventions to advance homeownership there is currently a declining trend in homeownership and an increase in renting experienced in South Africa. As the government does not have the resources to provide adequate housing to all South Africans, identifying the factors which attribute to non-subsidised homeownership will assist in implementing interventions and strategies to increase access to non-subsidised homeownership and reduce reliance on government subsidised housing. The main objective of this study was to determine the comparative odds of variables contributing to non-subsidised homeownership in South Africa from secondary data obtained from a South African household survey. Compared to the heuristic model, the following variables were found to align closely with the expectation created; non-subsidised homeownership attainment was most likely for households within high-income groups and least for households within the low-income groups, more likely for households who have access to credit than those without, more likely for households with no accounts in arrears than those with accounts in arrears, more likely for households with an ability to save than those without, most likely for households consisting of seven or more household members and least likely for single member households, most likely for households where FKP (Financially Knowledgeable Person) has completed a tertiary education level and least likely for households with primary not completed education levels, most likely for households where the FKP is older (aged 65 and older) and least likely for young FKP households (aged between 18 and 24), most likely for households residing in rural areas and least likely for households residing in metropolitan areas, most likely for female FKP households and least likely for male FKP households. Unexpectedly the regression model indicated that non-subsidised homeownership is most likely for households where the Financially Knowledgeable Person (FKP) is not economically active (for example pensioners) and least likely for employed households, most likely for households from the African population group and least likely for Indian households, most likely where the FKP is never married or single and least likely for separated or divorced FKP households (which is expected) and most likely for households residing in Limpopo (which is expected) and least likely for households residing Western Cape. / Financial accounting / M. Phil. (Accounting Science)

Um dos estudos observacionais mais difundidos e usados em epidemiologia veterinária é o estudo do tipo transversal. Sua popularidade ocorre por fatores como baixo custo e rapidez comparados com outros tipos de estudos, além de ajudar a estimar a prevalência de uma doença (desfecho) e postular fatores associados com o desfecho, que poderão ser confirmados como fatores causais em outros tipos de estudos epidemiológicos. Porém, este tipo de estudo apresenta dois importantes desafios: a dependência dos dados, muito frequente dada a típica estrutura populacional de animais dentro do mesmo rebanho ou fazenda e a escolha da medida de associação para desfechos binários, tão frequentes neste modelo de estudo. Com o objetivo de contribuir com a compreensão global da epidemiologia do aborto bovino associado à Neospora caninum tendo em conta a estrutura populacional, construiu-se um modelo misto com os dados de um estudo transversal realizado em duas regiões do Rio Grande do Sul. Usaram-se dados de 60 propriedades amostradas em duas regiões (noroeste e sudeste) e 1256 bovinos. A percentagem de aborto dentro de cada rebanho variou entre 1% e 30%. Vacas soropositivas tiveram 6,63 vezes mais chances de ter histórico de aborto (IC 95%: 4,41-13,20). As chances de uma vaca ter histórico de aborto foram 5,18 vezes maiores na região noroeste em relação à região sudeste (IC 95%: 1,83-20,80). Um coeficiente de correlação intraclasse de 16% foi estimado, indicando que 16% da variação da ocorrência de abortamentos não explicados pelos efeitos fixos foram devido as fazendas. Já na segunda parte deste trabalho, uma revisão sistemática foi realizada considerando um conjunto diverso de revistas e jornais com o objetivo de verificar os métodos estatísticos usados e a adequação das interpretações das medidas de associação estimadas em estudos transversais na área de medicina veterinária. Um total de 62 artigos foi avaliado. A revisão mostrou que, independentemente do nível de prevalência relatado no artigo, 96% deles empregou regressão logística e, portanto, estimaram razão de chances (RC). Nos artigos com prevalência superior a 10%, 23 deles fizeram uma interpretação adequada da RC como uma “razão de chances” ou simplesmente não fizeram uma interpretação direta da RC, enquanto 23 artigos interpretaram de forma inadequada a RC, considerando-a como risco ou probabilidade. Entre os artigos com prevalência inferior a 10%, apenas três interpretaram a RC como uma “razão de chances”, cinco interpretaram como risco ou probabilidade e em um, apesar de ter estimado a razão de prevalências (RP), foi interpretado de forma inadequada. Paralelamente, com o objetivo de exemplificar o uso de métodos estatísticos que estimam diretamente a razão de prevalências (RP), medida mais adequada para os estudos transversais, um conjunto de dados obtidos a partir de um estudo transversal sobre a ocorrência de anticorpos (AC) contra o vírus da diarreia viral bovina (BVDV) foi usado. Os AC foram medidos em amostras de tanque de leite de rebanhos leiteiros localizados no estado do Rio Grande do Sul, em que os possíveis fatores associados puderam ser avaliados. Entre os métodos utilizados, as maiores discrepâncias nas medidas de associação estimadas foram observadas com a regressão logística tomando-se como referência a regressão log-binomial. Finalmente, é importante que este tipo de desafio seja atendido pelos pesquisadores que realizam estudos transversais, ou seja, considerar a estrutura das populações nas análises, cuidado ao escolher o tipo de modelo estatístico empregado para desfecho binário e interpretação dos estimadores. / The commonest study design used in veterinary epidemiology is the cross-sectional study. Its popularity lies on the fact of the short time needed and low costs compared with other types of studies; moreover, this type of study estimates prevalence and associated factors, which may be elucidated as causal in another type of epidemiological studies. However, this type of study presents two major challenges: a very common dependence between data given the typical structure of the animal population, i.e., animals within herds or farms and the choice of measure of association for binary outcomes, frequently used in this type of study. In order to contribute to the understanding of the epidemiology of bovine abortion associated with Neospora caninum, a mixed model accounting for the hierarchical structure of cattle population using data from a cross-sectional study conducted in two regions (northwest and southeast) of Rio Grande do Sul was made. Data from 60 dairy herds and 1256 sampled cattle were used. The percentage of abortions in each herd ranged between 1% and 30%. Seropositive cows were 6.63 times more likely to have a history of abortion (95% CI: 4.41 to 13.20). The chances of a cow have a history of abortion were 5.18 times higher in the northwest comparing with the southeast region (95% CI: 1.83 to 20.80). An intraclass correlation coefficient (ICC) of 16% was estimated which means that 16% of the variation in abortion occurrence not explained by the fixed effects is due to farms. In the second part of this work, a systematic review was conducted considering a range of journals and newspapers in order to verify the statistical methods used and the adequacy of the interpretations of the measures of association estimated in cross-sectional studies from the veterinary medicine field. A total of 62 articles were revised. The review showed that, regardless of the reported prevalence, 96% of them employed logistic regression, therefore estimating odds ratio (OR). From the articles that reported prevalence rates above 10%, 23 of them did a proper interpretation of OR as an odds ratio, or simply did not make a direct interpretation of the OR, while 23 articles interpreted improperly the OR as a risk or probability. Among the articles that reported prevalence rates lower than 10%, only three interpreted the OR as an odds ratio, five interpreted as a risk or probability and only one, despite the estimated prevalence ratio (PR), it was improperly interpreted. Meanwhile, in order to exemplify the use of statistical methods to estimate directly the PR, the more appropriate measure of association in cross-sectional studies, a data set obtained from a cross-sectional study to estimate the occurrence of antibodies (AB) against bovine viral diarrhea virus (BVDV) in milk was used; AB were measured in bulk tank samples from dairy herds located in the state of Rio Grande do Sul, Brazil, and also possible associated factors were estimated. Among the methods used, major discrepancies in the measures of association estimated were observed with the logistic regression, comparing with the log-binomial regression. Finally, it is important that such challenges are met by the researchers that undertake cross-sectional studies.