A new source detection algorithm using FDR

The False Discovery Rate (FDR) method has recently been described by Miller et al (2001), along with several examples of astrophysical applications. FDR is a new statistical procedure due to Benjamini and Hochberg (1995) for controlling the fraction of false positives when performing multiple hypothesis testing. The importance of this method to source detection algorithms is immediately clear. To explore the possibilities offered we have developed a new task for performing source detection in radio-telescope images, Sfind 2.0, which implements FDR. We compare Sfind 2.0 with two other source detection and measurement tasks, Imsad and SExtractor, and comment on several issues arising from the nature of the correlation between nearby pixels and the necessary assumption of the null hypothesis. The strong suggestion is made that implementing FDR as a threshold defining method in other existing source-detection tasks is easy and worthwhile. We show that the constraint on the fraction of false detections as specified by FDR holds true even for highly correlated and realistic images. For the detection of true sources, which are complex combinations of source-pixels, this constraint appears to be somewhat less strict. It is still reliable enough, however, for a priori estimates of the fraction of false source detections to be robust and realistic.Comment: 17 pages, 7 figures, accepted for publication by A

Improving the Reliability and Modal Stability of High Power 870 nm AlGaAs CSP Laser Diodes for Applications to Free Space Communication Systems

The operating characteristics (power-current, beam divergence, etc.) and reliability assessment of high-power CSP lasers is discussed. The emission wavelength of these lasers was optimized at 860 to 880 nm. The operational characteristics of a new laser, the inverse channel substrate planar (ICSP) laser, grown by metalorganic chemical vapor deposition (MOCVD), is discussed and the reliability assessment of this laser is reported. The highlights of this study include a reduction in the threshold current value for the laser to 15 mA and a degradation rate of less than 2 kW/hr for the lasers operating at 60 mW of peak output power

A Bayesian Approach to Comparing Cosmic Ray Energy Spectra

A common problem in ultra-high energy cosmic ray physics is the comparison of energy spectra. The question is whether the spectra from two experiments or two regions of the sky agree within their statistical and systematic uncertainties. We develop a method to directly compare energy spectra for ultra-high energy cosmic rays from two different regions of the sky in the same experiment without reliance on agreement with a theoretical model of the energy spectra. The consistency between the two spectra is expressed in terms of a Bayes factor, defined here as the ratio of the likelihood of the two-parent source hypothesis to the likelihood of the one-parent source hypothesis. Unlike other methods, for example chi^2 tests, the Bayes factor allows for the calculation of the posterior odds ratio and correctly accounts for non-Gaussian uncertainties. The latter is particularly important at the highest energies, where the number of events is very small.Comment: 22 pages, 10 figures, accepted for publication in Ap

Prevalence and correlates of violence among South African high school learners in uMgungundlovu District municipality, KwaZulu-Natal, South Africa

Background. Young people grow up in homes and communities where many are exposed daily to crime and antisocial behaviours.Objective. To investigate the prevalence of violence and the demographic factors associated with such violence among South African (SA) high school learners in the uMgungundlovu District, KwaZulu-Natal, SA.Methods. In a cross-sectional study, we used stratified random sampling to select 16 schools in uMgungundlovu District. All Grade 10 high school learners (N=1 741) completed a self-administered questionnaire (Centers for Disease Control Youth Risk Behavior Survey). Data analysis was carried out using STATA 13 statistical software (Statacorp, USA).Results. Of the participants in this study, 420 (23.9%) had been bullied, 379 (21.7%) had missed school because of feeling unsafe, 468 (15.4%) had been involved in physical fights and 41 (2.4%) had carried weapons to school. There was a significant association between being in a physical fight and missing school (odds ratio (OR) 2.5, 95% confidence interval (CI) 1.9 - 3.3; p<0.001). There were higher odds of male learners carrying weapons than female learners (OR 5.9, 95% CI 2.0 - 15.0). Among learners living in rented rooms, the OR of feeling unsafe was 2.7 (95% CI 0.8 - 3.0), in an informal settlement the OR was 0.8 (95% CI 0.3 - 2.0) and in reconstruction and development programme houses it was 2.7 (95% CI 1.0 - 5.0), compared with learners residing in Zulu homesteads.Conclusion. Violence among learners attending high schools in uMgungundlovu District is a major problem and has consequences for both their academic and social lives. Urgent interventions are required to reduce the rates of violence among high school learners

Spatially Resolved Galaxy Star Formation and its Environmental Dependence I

We use the photometric information contained in individual pixels of 44,964 (0.019<z<0.125 and -23.5<M_r<-20.5) galaxies in the Fourth Data Release (DR4) of the Sloan Digital Sky Survey to investigate the effects of environment on galaxy star formation (SF). We use the pixel-z technique, which combines stellar population synthesis models with photometric redshift template fitting on the scale of individual pixels in galaxy images. Spectral energy distributions are constructed, sampling a wide range of properties such as age, star formation rate (SFR), dust obscuration and metallicity. By summing the SFRs in the pixels, we demonstrate that the distribution of total galaxy SFR shifts to lower values as the local density of surrounding galaxies increases, as found in other studies. The effect is most prominent in the galaxies with the highest star formation, and we see the break in the SFR-density relation at a local galaxy density of $\approx 0.05$(Mpc/h)$^{-3}$. Since our method allows us to spatially resolve the SF distribution within galaxies, we can calculate the mean SFR of each galaxy as a function of radius. We find that on average the mean SFR is dominated by SF in the central regions of galaxies, and that the trend for suppression of SFR in high density environments is driven by a reduction in this nuclear SF. We also find that the mean SFR in the outskirts is largely independent of environmental effects. This trend in the mean SFR is shared by galaxies which are highly star forming, while those which are weakly star forming show no statistically significant correlation between their environment and the mean SFR at any radius.Comment: 37 pages, 11 figures. Referee's comments included and matches version accepted for publication in the Astrophysical Journal. For high resolution figures, see http://www.phyast.pitt.edu/~welikala/pixelz/paper1

Significance of HbA1c levels in diabetic retinopathy extremes in South Africa

Background. Diabetic retinopathy (DR) is one of the leading causes of blindness in sub-Saharan Africa and globally, placing a huge disease burden on patients and the public health system. DR varies in severity from non-proliferative to proliferative DR (PDR).Objectives. Using a monitor of medium- to long-term blood glucose control, to determine the association between glycated haemoglobin (HbA1c) levels in patients with PDR and those with no DR.Methods. A prospective, cross-sectional study was conducted at McCord Provincial Eye Hospital in Durban, South Africa. We studied only patients diagnosed with diabetes mellitus (DM) for &gt;1 year who had either PDR or no DR, and compared their HbA1c levels. Patients with non-proliferative DR were not included.Results. Patients with PDR had significantly higher HbA1c levels than those with no DR. Patients with type 1 DM had higher HbA1c levels than patients with type 2 DM in both the PDR and no-DR groups. Older patients (&gt;70 years) had lower HbA1c levels than younger patients. Gender, race and duration of diabetes had no influence on HbA1c levels.Conclusions. PDR was associated with higher HbA1c in type 2 DM in all races and age groups and was independent of duration of disease. The trend was the same for type 1 DM, but significance could not be reached, probably because of small numbers in this subset of patients

Significance of HbA1c levels in diabetic retinopathy extremes in South Africa

Background. Diabetic retinopathy (DR) is one of the leading causes of blindness in sub-Saharan Africa and globally, placing a huge disease burden on patients and the public health system. DR varies in severity from non-proliferative to proliferative DR (PDR). Objectives. Using a monitor of medium-to long-term blood glucose control, to determine the association between glycated haemoglobin (HbA1c) levels in patients with PDR and those with no DR. Methods. A prospective, cross-sectional study was conducted at McCord Provincial Eye Hospital in Durban, South Africa. We studied only patients diagnosed with diabetes mellitus (DM) for &gt;1 year who had either PDR or no DR, and compared their HbA1c levels. Patients with non-proliferative DR were not included. Results. Patients with PDR had significantly higher HbA1c levels than those with no DR. Patients with type 1 DM had higher HbA1c levels than patients with type 2 DM in both the PDR and no-DR groups. Older patients (&gt;70 years) had lower HbA1c levels than younger patients. Gender, race and duration of diabetes had no influence on HbA1c levels. Conclusions. PDR was associated with higher HbA1c in type 2 DM in all races and age groups and was independent of duration of disease. The trend was the same for type 1 DM, but significance could not be reached, probably because of small numbers in this subset of patients