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Search Results: 1 - 10 of 147081 matches for " Rebecca F Grais "
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Optimisation of the T-square sampling method to estimate population sizes
Kristof Bostoen, Zaid Chalabi, Rebecca F Grais
Emerging Themes in Epidemiology , 2007, DOI: 10.1186/1742-7622-4-7
Abstract: There is a constant need to estimate population size and density for the purposes of planning resource requirements or assessing health needs. For reasons relating to timeliness, cost or practicality, data are often obtained through surveys that aim to collect representative samples. Public health specialists rely traditionally on detailed sample frames to survey populations. There are however many situations (such as those relating to displaced populations in emergencies) in which detailed sample frames are either unavailable or unfeasible. Only a small number of sampling methods are suitable for such situations.Ecological methods, which often do not require a detailed sample frame, can offer practical solutions to household sampling problems and are currently being explored. These methods include sequential sampling techniques to estimate prevalence or program coverage [1,2], capture-recapture techniques [3,4], adaptive sampling [5], T-Square sampling [6] and Catana's wandering quarter method [7] to estimate population size and density.One of the problems in validating and verifying sampling methods used in situations devoid of sampling frames is the difficulty in analysing the properties of the sampling methods [8]. Traditional optimisation of sampling methods is done using computationally intensive re-sampling techniques such as Monte Carlo (MC) or Latin Hypercube Sampling (LHS) simulations, while experimenting with different permutations of the parameters of the sampling method on simulated or real population data. Further, from a theoretical perspective, there are infinitely many scenarios (covering a wide distribution of household and individual data) for which the sampling method requires validation and verification.Mathematical Programming (MP) provides a powerful tool to optimise rigorously the properties of sampling methods [8]. The key advantage of MP is that it provides a more directed and less computing-intensive approach for optimisation compared to t
Don't spin the pen: two alternative methods for second-stage sampling in urban cluster surveys
Rebecca F Grais, Angela MC Rose, Jean-Paul Guthmann
Emerging Themes in Epidemiology , 2007, DOI: 10.1186/1742-7622-4-8
Abstract: Over time, the World Health Organization Expanded Program on Immunization (EPI) cluster survey design has become the default choice in the field to measure vaccination coverage and other indicators, even when a sampling frame is available. The cluster method was developed in the 1970s for immunization coverage in the USA and expanded for the smallpox eradication campaign later that decade [1,2]. In a two-stage cluster design, the first stage involves selection of clusters throughout the survey area; this is usually done proportionally to estimated population size. The next stage concerns selection of the first sample household within the cluster. The method has been described in detail in previous publications and manuals [3-8].Despite criticism and recommendations for its modification, [9-13] these are rarely taken into consideration and the 'original' methodology may be followed by default. In this paper, we focus on one problematic aspect of the cluster survey as it is often implemented: selection of households in the second stage. Although the original method called for a list of households to be selected at random from a list of all eligible households, this is not usually implemented, as such lists are rarely available, especially in settings where cluster surveys are often performed. Instead, variations on a "random walk" are widely used. In perhaps the most common implementation, survey teams select a random starting direction from a central location in the cluster by spinning a pen or bottle. Households lying on this transect from the center to the border of the cluster are counted and one of them is then chosen at random. Proximity selection is then used to select subsequent households as the "next nearest" until the desired sample size is reached."Spinning the pen" is often justified as a way to avoid costly and time-consuming listing of all households in the selected cluster in the absence of a sampling frame. As long as the starting point is selected ra
Challenges in measuring measles case fatality ratios in settings without vital registration
K Lisa Cairns, Robin Nandy, Rebecca F Grais
Emerging Themes in Epidemiology , 2010, DOI: 10.1186/1742-7622-7-4
Abstract: Measles, a highly infectious vaccine-preventable viral disease, is characterized by clustering of cases that occur during cyclical epidemics [1]. In many parts of the world, measles is also a seasonal disease with fewer cases found during the non-measles season [2]. Clinically, the infection is expressed as a maculopapular rash accompanied by fever and at least one of the three "c's": cough, coryza and conjunctivitis; virtually all cases of measles are clinically expressed [3,4]. Measles is a potentially fatal disease [1]. The World Health Organization (WHO) defines a measles-associated death as one occurring within 30 days of rash onset, not obviously due to another cause such as trauma [5].Historically, measles case fatality ratios (CFRs) have been reported to vary from 0.1% [1] in the developed world to as high as 30% among refugee populations [6,7]. Current estimates of CFRs used by WHO in endemic countries range between 0.05% - 6% [8-10]. Factors thought to affect CFR include age [11], intensity of exposure to measles virus (for which household crowding may be seen as a surrogate) [12], measles vaccination status [13], nutritional status [14], immunodeficiency [15] and access to appropriate case management [16]. Studies conducted in the late 1980 s demonstrated that supplementation of measles case-patients with vitamin A could decrease measles mortality by as much as 64% [17,18] leading to recommendations by WHO and United Nations Children's Fund (UNICEF) in 1987 to treat all measles case-patients with vitamin A in areas where measles CFRs were greater than 1% [19]. These recommendations, in conjunction with the rollout of Integrated Management of Childhood Illness (IMCI) guidelines in the mid 1990 s [20] which target pneumonia and diarrhea, might be anticipated to have decreased measles CFRs since the 1990 s. However, few data exist on the extent to which these interventions are used in health facilities, particularly in countries that are highly endemic for m
Description and consequences of sexual violence in Ituri province, Democratic Republic of Congo
Fran?oise Duroch, Melissa McRae, Rebecca F Grais
BMC International Health and Human Rights , 2011, DOI: 10.1186/1472-698x-11-5
Abstract: We describe the characteristics and consequences of sexual violence in Ituri province of Democratic Republic of Congo through the retrospective analysis of 2,565 patients who received medical care in the Médecins Sans Frontières sexual violence clinic in the capital of Ituri province, Bunia, between September 2005 and December 2006. Using a standardised questionnaire, we report patients' demographics, number and status of aggressor(s), forced detention and violent threats among other variables for all patients presenting for medical consultation after a sexually violent event during this period.Ninety-six percent of our cohort were female and 29.3% minors, 18-29 years was the most represented age group. Acts of sexual violence (n = 2,565) were reported to be mainly perpetrated by men with military affiliations (73%), although civilians were implicated in 21% of crimes. The attack was perpetrated by two or more persons in over 74% of cases and most commonly perpetrators were unknown armed males, (87.2%). Male victims accounted for 4% (n = 103) of our cohort. Forty-eight percent of our patients reported being attacked whilst performing daily domestic duties outside the home and 18% of victims being detained by their perpetrators, the majority of whom were held for less than 2 weeks (61.6%).The characteristics of sexually violent acts in Ituri province during this period cannot be simply explained as a 'weapon of war' as described in the literature, meaning the use of sexual violence within a military strategy where it is employed under the orders of a commander to harm a particular community. Whilst the majority of aggressions were by armed men there was an important proportion in which civilian perpetrators were implicated. This type of violence has become part of the general characteristics of violence in this war-torn population. Sometimes, as a means for some military factions to acquire remuneration with impunity and for some civilians, a means to counteract conf
A comparison of cluster and systematic sampling methods for measuring crude mortality
Rose,Angela MC; Grais,Rebecca F; Coulombier,Denis; Ritter,Helga;
Bulletin of the World Health Organization , 2006, DOI: 10.1590/S0042-96862006000400013
Abstract: objective: to compare the results of two different survey sampling techniques (cluster and systematic) used to measure retrospective mortality on the same population at about the same time. methods: immediately following a cluster survey to assess mortality retrospectively in a town in north darfur, sudan in 2005, we conducted a systematic survey on the same population and again measured mortality retrospectively. this was only possible because the geographical layout of the town, and the availability of a good previous estimate of the population size and distribution, were conducive to the systematic survey design. results: both the cluster and the systematic survey methods gave similar results below the emergency threshold for crude mortality (0.80 versus 0.77 per 10 000/day, respectively). the results for mortality in children under 5 years old (u5mr) were different (1.16 versus 0.71 per 10 000/day), although this difference was not statistically significant. the 95% confidence intervals were wider in each case for the cluster survey, especially for the u5mr (0.15-2.18 for the cluster versus 0.09-1.33 for the systematic survey). conclusion: both methods gave similar age and sex distributions. the systematic survey, however, allowed for an estimate of the town's population size, and a smaller sample could have been used. this study was conducted in a purely operational, rather than a research context. a research study into alternative methods for measuring retrospective mortality in areas with mortality significantly above the emergency threshold is needed, and is planned for 2006.
A comparison of cluster and systematic sampling methods for measuring crude mortality
Rose Angela MC,Grais Rebecca F,Coulombier Denis,Ritter Helga
Bulletin of the World Health Organization , 2006,
Abstract: OBJECTIVE: To compare the results of two different survey sampling techniques (cluster and systematic) used to measure retrospective mortality on the same population at about the same time. METHODS: Immediately following a cluster survey to assess mortality retrospectively in a town in North Darfur, Sudan in 2005, we conducted a systematic survey on the same population and again measured mortality retrospectively. This was only possible because the geographical layout of the town, and the availability of a good previous estimate of the population size and distribution, were conducive to the systematic survey design. RESULTS: Both the cluster and the systematic survey methods gave similar results below the emergency threshold for crude mortality (0.80 versus 0.77 per 10 000/day, respectively). The results for mortality in children under 5 years old (U5MR) were different (1.16 versus 0.71 per 10 000/day), although this difference was not statistically significant. The 95% confidence intervals were wider in each case for the cluster survey, especially for the U5MR (0.15-2.18 for the cluster versus 0.09-1.33 for the systematic survey). CONCLUSION: Both methods gave similar age and sex distributions. The systematic survey, however, allowed for an estimate of the town's population size, and a smaller sample could have been used. This study was conducted in a purely operational, rather than a research context. A research study into alternative methods for measuring retrospective mortality in areas with mortality significantly above the emergency threshold is needed, and is planned for 2006.
Does the Effectiveness of Control Measures Depend on the Influenza Pandemic Profile?
Solen Kernéis, Rebecca F. Grais, Pierre-Yves Bo?lle, Antoine Flahault, Elisabeta Vergu
PLOS ONE , 2008, DOI: 10.1371/journal.pone.0001478
Abstract: Background Although strategies to contain influenza pandemics are well studied, the characterization and the implications of different geographical and temporal diffusion patterns of the pandemic have been given less attention. Methodology/Main Findings Using a well-documented metapopulation model incorporating air travel between 52 major world cities, we identified potential influenza pandemic diffusion profiles and examined how the impact of interventions might be affected by this heterogeneity. Clustering methods applied to a set of pandemic simulations, characterized by seven parameters related to the conditions of emergence that were varied following Latin hypercube sampling, were used to identify six pandemic profiles exhibiting different characteristics notably in terms of global burden (from 415 to >160 million of cases) and duration (from 26 to 360 days). A multivariate sensitivity analysis showed that the transmission rate and proportion of susceptibles have a strong impact on the pandemic diffusion. The correlation between interventions and pandemic outcomes were analyzed for two specific profiles: a fast, massive pandemic and a slow building, long-lasting one. In both cases, the date of introduction for five control measures (masks, isolation, prophylactic or therapeutic use of antivirals, vaccination) correlated strongly with pandemic outcomes. Conversely, the coverage and efficacy of these interventions only moderately correlated with pandemic outcomes in the case of a massive pandemic. Pre-pandemic vaccination influenced pandemic outcomes in both profiles, while travel restriction was the only measure without any measurable effect in either. Conclusions Our study highlights: (i) the great heterogeneity in possible profiles of a future influenza pandemic; (ii) the value of being well prepared in every country since a pandemic may have heavy consequences wherever and whenever it starts; (iii) the need to quickly implement control measures and even to anticipate pandemic emergence through pre-pandemic vaccination; and (iv) the value of combining all available control measures except perhaps travel restrictions.
A Look Back at an Ongoing Problem: Shigella dysenteriae Type 1 Epidemics in Refugee Settings in Central Africa (1993–1995)
Solen Kernéis, Philippe J. Guerin, Lorenz von Seidlein, Dominique Legros, Rebecca F. Grais
PLOS ONE , 2009, DOI: 10.1371/journal.pone.0004494
Abstract: Background Shigella dysenteriae type 1 (Sd1) is a cause of major dysentery outbreaks, particularly among children and displaced populations in tropical countries. Although outbreaks continue, the characteristics of such outbreaks have rarely been documented. Here, we describe the Sd1 outbreaks occurring between 1993 and 1995 in 11 refugee settlements in Rwanda, Tanzania and Democratic Republic of the Congo (DRC). We also explored the links between the different types of the camps and the magnitude of the outbreaks. Methodology/Principal Findings Number of cases of bloody diarrhea and deaths were collected on a weekly basis in 11 refugee camps, and analyzed retrospectively. Between November 1993 and February 1995, 181,921 cases of bloody diarrhea were reported. Attack rates ranged from 6.3% to 39.1% and case fatality ratios (CFRs) from 1.5% to 9.0% (available for 5 camps). The CFRs were higher in children under age 5. In Tanzania where the response was rapidly deployed, the mean attack rate was lower than in camps in the region of Goma without an immediate response (13.3% versus 32.1% respectively). Conclusions/Significance This description, and the areas where data is missing, highlight both the importance of collecting data in future epidemics, difficulties in documenting outbreaks occurring in complex emergencies and most importantly, the need to assure that minimal requirements are met.
Prognostic Accuracy of WHO Growth Standards to Predict Mortality in a Large-Scale Nutritional Program in Niger
Nathanael Lapidus,Francisco J Luquero,Valérie Gaboulaud,Susan Shepherd,Rebecca F Grais
PLOS Medicine , 2009, DOI: 10.1371/journal.pmed.1000039
Abstract: Background Important differences exist in the diagnosis of malnutrition when comparing the 2006 World Health Organization (WHO) Child Growth Standards and the 1977 National Center for Health Statistics (NCHS) reference. However, their relationship with mortality has not been studied. Here, we assessed the accuracy of the WHO standards and the NCHS reference in predicting death in a population of malnourished children in a large nutritional program in Niger. Methods and Findings We analyzed data from 64,484 children aged 6–59 mo admitted with malnutrition (<80% weight-for-height percentage of the median [WH]% [NCHS] and/or mid-upper arm circumference [MUAC] <110 mm and/or presence of edema) in 2006 into the Médecins Sans Frontières (MSF) nutritional program in Maradi, Niger. Sensitivity and specificity of weight-for-height in terms of Z score (WHZ) and WH% for both WHO standards and NCHS reference were calculated using mortality as the gold standard. Sensitivity and specificity of MUAC were also calculated. The receiver operating characteristic (ROC) curve was traced for these cutoffs and its area under curve (AUC) estimated. In predicting mortality, WHZ (NCHS) and WH% (NCHS) showed AUC values of 0.63 (95% confidence interval [CI] 0.60–0.66) and 0.71 (CI 0.68–0.74), respectively. WHZ (WHO) and WH% (WHO) appeared to provide higher accuracy with AUC values of 0.76 (CI 0.75–0.80) and 0.77 (CI 0.75–0.80), respectively. The relationship between MUAC and mortality risk appeared to be relatively weak, with AUC = 0.63 (CI 0.60–0.67). Analyses stratified by sex and age yielded similar results. Conclusions These results suggest that in this population of children being treated for malnutrition, WH indicators calculated using WHO standards were more accurate for predicting mortality risk than those calculated using the NCHS reference. The findings are valid for a population of already malnourished children and are not necessarily generalizable to a population of children being screened for malnutrition. Future work is needed to assess which criteria are best for admission purposes to identify children most likely to benefit from therapeutic or supplementary feeding programs.
Measles vaccination in humanitarian emergencies: a review of recent practice
Rebecca F Grais, Peter Strebel, Peter Mala, John Watson, Robin Nandy, Michelle Gayer
Conflict and Health , 2011, DOI: 10.1186/1752-1505-5-21
Abstract: We searched the published literature for articles published from January 1, 1998 to January 1, 2010 reporting on measles in emergencies. As definitions and concepts of emergencies vary and have changed over time, we chose to consider any context where an application for either a Consolidated Appeals Process or a Flash Appeal to the UN Central Emergency Revolving Fund (CERF) occurred during the period examined. We included publications from countries irrespective of their progress in measles control as humanitarian emergencies may occur in any of these contexts and as such, guidance applies irrespective of measles control goals.Of the few well-documented epidemic descriptions in humanitarian emergencies, the age range of cases is not limited to under 5 year olds. Combining all data, both from preventive and outbreak response interventions, about 59% of cases in reports with sufficient data reviewed here remain in children under 5, 18% in 5-15 and 2% above 15 years. In instances where interventions targeted a reduced age range, several reports concluded that the age range should have been extended to 15 years, given that a significant proportion of cases occurred beyond 5 years of age.Measles outbreaks continue to occur in humanitarian emergencies due to low levels of pre-existing population immunity. According to available published information, cases continue to occur in children over age 5. Preventing cases in older age groups may prevent younger children from becoming infected and reduce mortality in both younger and older age groups.Humanitarian emergencies occur in situations of conflict, war or civil disturbance, natural disasters, food insecurity or other crises resulting in disruptions that overwhelm national capacities and require international assistance [1]. The health needs of children and adolescents in humanitarian emergencies are critical to the success of relief efforts and reduction in mortality. Measles has been one of the major causes of child deat
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