Papers by Francois Steffens
Technometrics, 1973
The two-sided bivariate t-test for simultaneous comparison of two treatments with a control is st... more The two-sided bivariate t-test for simultaneous comparison of two treatments with a control is studied. The problem of allocating a given number of observations to the treatments and the control is discussed, and it is concluded that, in the absence of preferences, a general purpose rule would be to choose the sample sizes equal. A table is presented which gives the sample size required to ensure that the null hypothesis will be rejected with at least a prescribed probability, given the significance level and the standardized differences between the treatments and the control. Similar tables are provided for use when the object is to ensure that both treatments means will be found significantly different from the control mean with at least a given probability, or to ensure that a specified treatment will be found significant with at least a given probability.
I am indebted to my supervisor, Jonathan Levin, for the stimulating discussions and comments on s... more I am indebted to my supervisor, Jonathan Levin, for the stimulating discussions and comments on successive drafts of this dissertation and also for providing me with a suitable data set for analysis. Many thanks to Prof. Steffens for his valuable comments. To my family, friends and colleagues who supported me throughout my years of study, I am grateful.
Journal of the American Statistical Association, 1969
Let t1 and t12 be two independent normally distributed random variables with zero means and equal... more Let t1 and t12 be two independent normally distributed random variables with zero means and equal variances, each divided by a common estimate of the standard deviations. The joint distribution of t1, t2 is a bivariate Student t-distribution. The integral of the joint frequency function over certain types of critical regions in the (t1, t2)-plane may be written as the
South African medical journal = Suid-Afrikaanse tydskrif vir geneeskunde, 1975
The average dark adaptation time for newly recruited Black mineworkers is longer than that report... more The average dark adaptation time for newly recruited Black mineworkers is longer than that reported for Whites. While this longer dark adaptation time may result from a number of pathological conditions, its most likely cause is a deficiency of vitamin A in the diet. A study was conducted in which the vitamin A level of the blood and the dark adaptation times were correlated in a group of Black mineworkers upon arrival and again after 4 - 6 months continuous work underground in a gold mine. The study indicated that a very marked decrease occurred in the miners' serum vitamin A level between the time of arrival at the mine and the second examination. This difference was found to be highly significant. This decrease was accompanied by an increase in mean dark adaptation time, which was significant at the 5% level. These observations accord with the findings of a previous study and are indicative of an inadequate dietary intake of vitamin A during the period when the miners were on...
South African Journal of Geology, 1985
Hygroscopic seeding was conducted in the sixties and early seventies in the USA, but since then, ... more Hygroscopic seeding was conducted in the sixties and early seventies in the USA, but since then, most cloud seeding efforts have used glaciogenic seeding materials, usually silver iodide and dry ice. Very little research has since been conducted on hygroscopic seeding in the USA and very few operational programs use hygroscopic materials. The reasons for this are the high amounts of seeding material needed and the usual corrosive character of these materials and the usual corrosive nature of these materials. Recently, an ongoing summertime convective cloud seeding research program in South Africa started using hygroscopic flares for seeding convective clouds for rainfall enhancement. After one season of randomized seeding highly significant results were obtained indicating that seeded clouds on average produce 100 ktons more water than unseeded clouds as derived from radar data. These results spurred intensive physical studies to determine the effect of the seeding material on the p...
South African Journal of Botany
Abstract This is the first study documenting the distribution and population biology of the Endan... more Abstract This is the first study documenting the distribution and population biology of the Endangered Aloe lettyae, an endemic to the highly threatened Woodbush Granite Grassland (WGG) in Limpopo Province, South Africa. We documented 19 A. lettyae populations and calculated the total area of occupied habitat at 17.5 ha within its extent of occurrence of 123 km2. Most populations were clustered on the south western side of the WGG, with all known localities less than 40 km apart in this severely fragmented vegetation type. Population size varied from 10 to 6547 plants, with ∼10,800 individuals estimated in total. Plant size and life history stage demographics were determined in seven of the 19 currently known A. lettyae populations, including two large populations, as well as a high- and a low-lying population, and constituted a representative sample of the entire geographical range of the species. For plant size, four metrics (number of leaves, number of leaf layers, height and diameter of the leaf rosette) were recorded for each sampled A. lettyae plant, and the presence of an inflorescence indicated its reproductive status. By means of the classification tree technique CHAID (Chi-square Automatic Interaction Detection), we determined that of the four plant size metrics, the ‘number of leaves’ predicts A. lettyae’s life history stages and fecundity most accurately, and revealed significant differences in the life history stage structure of the seven surveyed populations. Five populations occurring in relatively undisturbed WGG fragments, including the two large populations, comprised high proportions of adult individuals (77–89%), while only few A. lettyae juveniles were found (3–14%). By contrast, the relatively high percentage of juveniles (25–58%) and low percentage of adults (32–63%) observed in two small populations found in a degraded WGG fragment and in non-natural habitat respectively, appeared atypical. The percentage of subadults varied little across the surveyed populations (8–13%). In slow-growing, long-lived species such as A. lettyae, the dominance of adult individuals with a high probability of survival may be considered as indicative of stable populations. This study provides baseline A. lettyae population data for long-term demographic monitoring which will aid management and conservation of the range restricted, Endangered A. lettyae.
Transactions of the Royal Society of Tropical Medicine and Hygiene, 1970
Abstract A computer-simulated model which is described here by means of an abbreviated flow diagr... more Abstract A computer-simulated model which is described here by means of an abbreviated flow diagram has been designed to express in quantitative terms the factors which play a role in the transmission cycle of an arbovirus. A frequency distribution is assigned to the interaction between the vertebrate host and insect vector, and in this respect the model differs from previous strictly deterministic models. The results (with hypothetical values for the variables) indicate that the most critical factor in the cycle is the survival probability of the vector, whereas the population dynamics of the vertebrate host have less effect, provided that a reasonably large numbers of animal are present. A field study of the cycle of Chikungunya virus is in progress and some of the findings have been used in the examples presented here.
Journal of Applied Physiology
Journal of The American Statistical Association, 1969
Let t1 and t12 be two independent normally distributed random variables with zero means and equal... more Let t1 and t12 be two independent normally distributed random variables with zero means and equal variances, each divided by a common estimate of the standard deviations. The joint distribution of t1, t2 is a bivariate Student t-distribution. The integral of the joint frequency function over certain types of critical regions in the (t1, t2)-plane may be written as the
Technometrics, 1973
The two-sided bivariate t-test for simultaneous comparison of two treatments with a control is st... more The two-sided bivariate t-test for simultaneous comparison of two treatments with a control is studied. The problem of allocating a given number of observations to the treatments and the control is discussed, and it is concluded that, in the absence of preferences, a general purpose rule would be to choose the sample sizes equal. A table is presented which gives the sample size required to ensure that the null hypothesis will be rejected with at least a prescribed probability, given the significance level and the standardized differences between the treatments and the control. Similar tables are provided for use when the object is to ensure that both treatments means will be found significantly different from the control mean with at least a given probability, or to ensure that a specified treatment will be found significant with at least a given probability.
South African medical journal = Suid-Afrikaanse tydskrif vir geneeskunde, Jan 24, 1975
The average dark adaptation time for newly recruited Black mineworkers is longer than that report... more The average dark adaptation time for newly recruited Black mineworkers is longer than that reported for Whites. While this longer dark adaptation time may result from a number of pathological conditions, its most likely cause is a deficiency of vitamin A in the diet. A study was conducted in which the vitamin A level of the blood and the dark adaptation times were correlated in a group of Black mineworkers upon arrival and again after 4 - 6 months continuous work underground in a gold mine. The study indicated that a very marked decrease occurred in the miners' serum vitamin A level between the time of arrival at the mine and the second examination. This difference was found to be highly significant. This decrease was accompanied by an increase in mean dark adaptation time, which was significant at the 5% level. These observations accord with the findings of a previous study and are indicative of an inadequate dietary intake of vitamin A during the period when the miners were on...
Journal of the American Statistical Association, 1969
Transactions of the Royal Society of Tropical Medicine and Hygiene 104 (2010) 364367 ... Transac... more Transactions of the Royal Society of Tropical Medicine and Hygiene 104 (2010) 364367 ... Transactions of the Royal Society of Tropical Medicine and Hygiene ... Differentiation of Leishmania (Viannia) panamensis and Leishmania (V.) ... Ana Margarita Montalvo Alvareza,∗, Jorge ...
Journal of Retailing and Consumer Services
International Journal of Consumer Studies, 2009
In many emerging economies and developing countries, comprising consumers from different cultures... more In many emerging economies and developing countries, comprising consumers from different cultures and with varying degrees of sophistication (knowledge and skill) concerning consumer protection, the promotion of consumers' rights to develop a consumeroriented culture remains a very big challenge. One way of protecting the consumer, especially the consumer that has not been fully socialized to execute informed decisions when purchasing expensive durables, is by establishing a redress environment that would ensure fair redress as well as an understanding and appreciation of the consumer. Manufacturers and retailers are often not aware of the performance failures that consumers experience concerning their products because many people do not communicate their dissatisfactions to them. However, unless and until manufacturers and retailers fully comprehend their customers' complaint behaviour, their reasons for engaging in specific complaint behaviour and the reasoning (cognitive processes) and emotional processes behind their behaviour, they will not recognize the link between complaint handling and customer loyalty and profits.
The Pharmacogenomics Journal
Pharmacogenomics, 2017
Therapy with low-dose amitriptyline is commonly used to treat painful diabetic peripheral neuropa... more Therapy with low-dose amitriptyline is commonly used to treat painful diabetic peripheral neuropathy. There is a knowledge gap, however, regarding the role of variable CYP2D6-mediated drug metabolism and side effects (SEs). We aimed to generate pilot data to demonstrate that SEs are more frequent in patients with variant CYP2D6 alleles. To that end, 31 randomly recruited participants were treated with low-dose amitriptyline for painful diabetic peripheral neuropathy and their CYP2D6 gene sequenced. Patients with predicted normal or ultra-rapid metabolizer phenotypes presented with less SEs compared with individuals with decreased CYP2D6 activity. Hence, CYP2D6 genotype contributes to treatment outcome and may be useful for guiding drug therapy. Future investigations in a larger patient population are planned to support these preliminary findings.
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Papers by Francois Steffens