Now showing 1 - 10 of 19
  • Publication
    Comparative Analysis of Swept-Source Optical Coherence Tomography and Partial Coherence Interferometry Biometers in the Prediction of Cataract Surgery Refractive Outcomes
    Purpose: To compare the accuracy of pre-operative corneal measurements obtained with four devices, and the refractive outcomes of two optical biometers. Setting: Private practice. Design: Retrospective. Methods: Data taken from biometric measurements on 299 consecutive eyes prior to cataract surgery were retrospectively analyzed using the Argos SS-Optical Biometer and the Lenstar LS900 PCI optical biometer. As part of the standard cataract surgery pre-operative exam, patients also underwent placido disk topography and Scheimpflug tomography. Keratometry, anterior chamber depth, corneal diameter, pupil diameter, central corneal thickness and axial length were all measured. The comparable measurements were compared. Finally, for those eyes where cataract surgery was performed, the post-operative refractive results were compared to the predictive results of the two biometers. Results: The SS-OCT Argos was able to measure all eyes, while five eyes could not be measured with the Lenstar LS900 PCI. Axial length measurements were performed only with the Argos and Lenstar devices. The eyes that could not be measured by the Lenstar LS900 PCI included dense grade IV nuclear sclerosis and large posterior subcapsular cataracts. In the primary endpoints, there was strong correlation between the Argos and the Lenstar devices in eyes with an axial length between 20 and 30 mm. Conclusion: The predictive accuracies of the Argos Optical Biometer and Lenstar LS900 PCI are similar, except in medium and long eyes, in which the predictive accuracy of Argos SS-OCT biometry was higher. The Argos system was found easier to use by technicians when compared to the other biometry devices.
      225Scopus© Citations 8
  • Publication
    Epidemic modelling of bovine tuberculosis in cattle herds and badgers in Ireland
    Bovine tuberculosis, a disease that affects cattle and badgers in Ireland, was studied via stochastic epidemic modeling using incidence data from the Four Area Project (Griffin et al., 2005). The Four Area Project was a large scale field trial conducted in four diverse farming regions of Ireland over a five-year period (1997-2002) to evaluate the impact of badger culling on bovine tuberculosis incidence in cattle herds. Based on the comparison of several models, the model with no between-herd transmission and badger-to-herd transmission proportional to the total number of infected badgers culled was best supported by the data. Detailed model validation was conducted via model prediction, identifiability checks and sensitivity analysis. The results suggest that badger-to-cattle transmission is of more importance than between-herd transmission and that if there was no badger-to-herd transmission, levels of bovine tuberculosis in cattle herds in Ireland could decrease considerably.
      137
  • Publication
    A neural network analysis of Lifeways cross-generation imputed data
    (BioMed Central, 2018-12-14)
    Objectives: Neural networks are a powerful statistical tool that use nonlinear regression type models to obtain predictions. Their use in the Lifeways cross-generation study that examined body mass index (BMI) of children, among other measures, is explored here. Our aim is to predict the BMI of children from that of their parents and maternal and paternal grandparents. For comparison purposes, linear models will also be used for prediction. A complicating factor is the large amount of missing data. The missing data may be imputed and we examine the effects of different imputation methods on prediction. An analysis using neural networks (and also linear models) that uses all available data without imputation is also carried out, and is the gold standard by which the analyses with imputed data sets are compared. Results: Neural network models performed better than linear models and can be used as a data analytic tool to detect nonlinear and interaction effects. Using neural networks the BMI of a child can be predicted from family members to within roughly 2.84 units. Results between the imputation methods were similar in terms of mean squared error, as were results based on imputed data compared to un-imputed data.
      332
  • Publication
    A comparison of sampling grids, cut off distance and type of residuals in parametric variogram estimation
    (Taylor and Francis, 2016-11-16) ;
    In spatial statistics, the correct identification of a variogram model when fitted to an empirical variogram depends on many factors. Here, simulation experiments show fitting based on the variogram cloud is preferable to that based on Matheron's and Cressie–Hawkins empirical variogram estimators. For correct model specification, a number of models should be fitted to the empirical variogram using a grid of cut-off values, and recommendations are given for best choice. A design where roughly half the maximum distance between points equals the practical range works well for correct variogram identification of any model, with varying nugget sizes and sample sizes.
      379Scopus© Citations 5
  • Publication
    Anisotropic spatial clustering of TB in cattle - the implications for control policy
    (Elsevier, 2011)
    Bovine TB is a disease that affects cattle and the wildlife badger, species Meles meles, in Ireland and the UK, and badgers have been implicated in the spread of the disease in cattle. Efforts to eradicate the disease that have included localized badger culling, have not been successful. In a study to understand how the disease spreads, Kelly and More [1] determined that the disease spatially clusters in cattle herds and estimated the practical spatial ranges at which this occurs. We extend this work by examining possible anisotropy in clustering and the consequences for TB control policy.
      312
  • Publication
    Body mass index and height over three generations: evidence from the Lifeways cross-generational cohort study
    Background: Obesity and its measure of body mass index are strongly determined by parental body size. Debate continues as to whether both parents contribute equally to offspring body mass which is key to understanding the aetiology of the disease. The aim of this study was to use cohort data from three generations of one family to examine the relative maternal and paternal associations with offspring body mass index and how these associations compare with family height to demonstrate evidence of genetic or environmental cross-generational transmission. Methods: 669 of 1082 families were followed up in 2007/8 as part of the Lifeways study, a prospective observational cross-generation linkage cohort. Height and weight were measured in 529 Irish children aged 5 to 7 years and were self-reported by parents and grandparents. All adults provided information on self-rated health, education status, and indicators of income, diet and physical activity. Associations between the weight, height, and body mass index of family members were examined with mixed models and heritability estimates computed using linear regression analysis. Results: Self-rated health was associated with lower BMI for all family members, as was age for children. When these effects were accounted for evidence of familial associations of BMI from one generation to the next was more apparent in the maternal line. Heritability estimates were higher (h2 = 0.40) for mother-offspring pairs compared to father-offspring pairs (h2 = 0.22). In the previous generation, estimates were higher between mothersparents (h2 = 0.54-0.60) but not between fathers-parents (h2 = -0.04-0.17). Correlations between mother and offspring across two generations remained significant when modelled with fixed variables of socioeconomic status, health, and lifestyle. A similar analysis of height showed strong familial associations from maternal and paternal lines across each generation. Conclusions: This is the first family cohort study to report an enduring association between mother and offspring BMI over three generations. The evidence of BMI transmission over three generations through the maternal line in an observational study corroborates the findings of animal studies. A more detailed analysis of geno and phenotypic data over three generations is warranted to understand the nature of this maternal-offspring relationship.
      393Scopus© Citations 89
  • Publication
    Happy birthday? An observational study
    Background: Previous studies show contradictory findings on the relationship between birthday and deathday, in particular whether people postpone death until after their birthday. We examine the phenomenon in eight groups of famous people. Methods: Birthday and deathday for the following groups were recorded: British prime ministers, US presidents, Academy Award best actor, best female actor, best director, Nobel Prize winners, Wimbledon men’s and ladies' singles winners, all from when records began. For each group, the difference in days between the deathday and birthday was calculated. Under the hypothesis of no association, one can expect the difference to have a uniform distribution. This is assessed using goodness-of-fit tests on a circle. Results: All groups showed some departure from the uniform and it occurred around the birthday in all groups. British prime ministers, US presidents, Academy Award actors and directors, Nobel Prize winners and Wimbledon men show a ’dip' in deaths around the birthday. The length of the ’dip' varied between the groups and so they gave different p-values on different test statistics. For Academy Award female actors and Wimbledon ladies, there was rise in deaths before and after birthday. When Nobel Prize winners were subdivided into their categories, Science and Literature had a ’dip' around the birthday, but not other categories. Conclusions: We conclude ’something' happens to deathday around the birthday. Some groups of famous people show a ’dip' in death rate around the birthday while for others, particularly women, the association is in the opposite direction.
      565Scopus© Citations 1
  • Publication
    Towards reliable spatial prediction
    (ECOSTA Econometrics and Statistics, 2018-12-16) ;
    Estimation of the variogram and associated parameters in spatial analysis is important for assessing spatial dependence and in predicting values of the measured variable at unsampled locations i.e. kriging. A simulation study is implemented to compare the performance of (i) Gaussian restricted maximum likelihood (REML) estimation, (ii) curve-fitting by ordinary least squares and (iii) nonparametric Shapiro-Botha estimation for estimating the covariance structure of a stationary Gaussian spatial process and a spatial process with t-distributed margins. Processes with Matern covariance functions are considered and the parameters estimated are the nugget, partial sill and practical range. Both parametric and nonparametric bootstrap distributions of the estimators are computed and compared to the true marginal distributions of the estimators. Gaussian REML is the estimator of choice for both Gaussian and t-distributed data and all choices of Matern variogram. However, accurate estimation of the Matern shape parameter is critical to achieving a good fit while this does not affect the Shapiro-Botha estimator. The parametric and nonparametric bootstrap both performed well, the latter being better for the Shapiro-Botha estimates. A numerical example, obtained from environmental monitoring, is included to illustrate the application of the methods and the bootstrap.
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  • Publication
    A simulation comparison of estimators of spatial covariance parameters and associated bootstrap percentiles
    (UCLA Department of Statistics, 2018-09) ;
    A simulation study is implemented to study estimators of the covariance structure of a stationary Gaussian spatial process and a spatial process with t-distributed margins. The estimators compared are Gaussian restricted maximum likelihood (REML) and curve-fitting by ordinary least squares and by the nonparametric Shapiro-Botha approach. Processes with Matérn covariance functions are considered and the parameters estimated are the nugget, partial sill and practical range. Both parametric and nonparametric bootstrap distributions of the estimators are computed and compared to the true marginal distributions of the estimators. Gaussian REML is the estimator of choice for both Gaussian and t-distributed data and all choices of the Matérn covariance structure. However, accurate estimation of the Matérn shape parameter is critical to achieving a good fit while this does not affect the Shapiro-Botha estimator. The parametric bootstrap performed well for all estimators although it tended to be biased downward. It was slightly better than the nonparametric bootstrap for Gaussian data, equivalent to it for t-distributed data and worse overall for the Shapiro-Botha estimates. A numerical example, obtained from environmental monitoring, is included to illustrate the application of the methods and the bootstrap.
      606
  • Publication
    Epithelium-on Corneal Cross-linking for Progressive Keratoconus: Two-year Outcomes
    (Jaypee Brothers Medical Publishing, 2018-12) ; ;
    Corneal cross-linking (CXL) has been established as a successful treatment tool for the treatment of progressive keratoconus in terms of slowing or halting progressive corneal steepening and thinning and even on some occasions, reversing the steepening. To date the Dresden epithelium-off protocol is regarded as the gold standard and the epithelium-on (epi-on) approaches have met with less success. Both doctors and patients would welcome an epi-on CXL procedure that provided good outcomes as the morbidity with epi-on CXL is so much less and the safety is enhanced. Patient comfort is greater with the epi-on techniques when compared to epi-off. This study looked at 82 eyes that had documented progression of keratoconus and then underwent epi-on CXL using the CXLO system. The results show that corneal steepening can be halted and even reversed over a 2-year follow-up period with no complications noted. Over the 24 months post treatment on average there was a decrease in all keratometry values, BAD and ISV when compared to before treatment with IHD being marginally increased. Further studies over a longer follow-up period are required but recent publications using the same approach are validating the findings seen in this study.
      426