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Discographic, MRI and psychosocial determinants of low back pain disability and remission: a prospective study in subjects with benign persistent back pain.

BACKGROUND CONTEXT: A range of morphologic and psychosocial variables has been suggested as risk factors for serious low back pain (LBP) illness. Although the relative contributions of structural and psychosocial variables are intensely debated, the validity of differing hypotheses has proven difficult to test because the incidence of serious disabling LBP illness is low in healthy subjects. These factors dictate the requirement for large sample sizes, extensive structural imaging and extended longitudinal study. Previous studies included either small cohorts with intensive imaging testing or large population studies that do not establish a detailed morphologic baseline. PURPOSE: To establish, using a strict patient sample design, the relative contribution of structural and psychosocial determinants of serious LBP illness among subjects with no previous LBP disability or clinical LBP illness. STUDY DESIGN/SETTING: A prospective, longitudinal study of subjects with high risk factors for serious LBP as determined by structural and psychosocial characteristics. PATIENT SAMPLE: One hundred subjects with known mild persistent low back pain and a 2:1 ratio of chronic (non-lumbar) pain syndrome were recruited from a study population with a predisposition to disc degenerative disease, to undergo baseline examination, testing and 5-year follow-up. OUTCOME MEASURES: Observations were made at 6-month intervals over 4 to 6 years (mean, 5.3) for the after primary outcomes measures: episodes of serious back pain (visual analogue scale [VAS] > or =6), episodes of occupational disability less than 1 week, episodes of occupational disability for 1 week, remission episodes of all back pain symptoms at least 6 months and medical visits primarily for LBP evaluation and treatment. METHODS: Lumbar magnetic resonance imaging (MRI), lumbar provocative discography (in psychometrically normal subjects), physical examinations, medical and work histories and psychometric testing were performed at baseline. Imaging and psychometric testing were graded by blinded examiners. A scripted interview was conducted every 6 months during follow-up by independent research assistants who also were blinded to patient baseline data. The interview covered interval medical, occupational and accident or injury histories. RESULTS: Psychosocial variables strongly predicted both long- and short-term disability events, duration and health-care visits for LBP problems (p<0.0001-0.004). The likelihood of a sustained remission from the baseline persistent (subclinical) LBP appeared to be linked to occupation factors (leaving a heavy labor occupation; p=0.0001), neurophysiologic variables (chronic nonlumbar pain; p=0.0002) and psychometric profiles at baseline (DRAM and FABQ-PA; p=0.003-0.002). Of the structural findings measured only moderate or severe Modic changes of the vertebral end plate were weakly associated with an adverse outcome. A positive provocative discogram at baseline did not predict any future adverse event. CONCLUSION: The development of serious LBP disability in a cohort of subjects with both structural and psychosocial risk factors was strongly predicted by baseline psychosocial variables. Structural variables on both MRI and discography testing at baseline had only weak association with back pain episodes and no association with disability or future medical care.

Adult↗
PubMed2006

Reply to comments on "Derivation of numerical values for the World Health Organization guidelines for recreational waters".

The contribution addressed reveals an optimistic design philosophy likely to systematically underestimate risk in epidemiologic studies into the health effects of bathing water exposures. The authors seem to recommend that data on the 'exposure' measure (i.e. water quality) in such studies should be acquired in a similar manner to that used for regulatory sampling. This approach may compromise the quality of the epidemiologic investigations undertaken. It may result in imprecise estimates of exposure because it ignores the fact that regulatory timescales and spatial resolution (even if artificially compressed to a bathing day) can mask large spatial and temporal variability in water quality. If this variability is ignored by taking some mean value and attributing that to all of those exposed in a period at a study location, many bathers may be misclassified and the studies may be biased to a 'no-effect' conclusion. A more appropriate approach is to maximise the precision of the epidemiologic investigations by measurement of individual exposure (or water quality) at the place and time of the exposure, as has been done in randomised volunteer studies in the UK and Germany. The precise epidemiologic relationships linking 'exposure' with 'illness' can then be related to the probability of exposure to particular water quality by a 'normal bather' using the known probability distribution of the exposure variable (i.e. faecal indicator concentration) in the regulated bathing waters. We suggest that any research protocol where poor sampling design for water quality assessment is justified because regulatory monitoring is equally imprecise may be fundamentally flawed. The rationale for this assessment is that the epidemiology is the starting point and evidence-base for 'standards'. If precision is not maximised at this stage in the process it compromises the credibility of the standards design process. The negative effects of the approach advocated in this 'comment' are illustrated using published research findings used to derive the figures illustrated in Wymer et al. [2005. Comment on derivation of numerical values for the World Health Organization guidelines for recreational waters. Water Research 39, 2774-2777].

Environmental Exposure↗
PubMed2002

Prevalence of selected maternal behaviors and experiences, Pregnancy Risk Assessment Monitoring System (PRAMS), 1999.

PROBLEM/CONDITION: Various maternal behaviors and experiences before, during, and after pregnancy (e.g., unintended pregnancy, late entry into prenatal care, cigarette smoking, not breast-feeding) are associated with adverse health outcomes for both the mother and the infant. Information regarding maternal behaviors and experiences is needed to monitor trends, to enhance the understanding of the relations between behaviors and health outcomes, to plan and evaluate programs, to direct policy decisions, and to monitor progress toward Healthy People 2000 and 2010 objectives. REPORTING PERIOD COVERED: This report covers data from 1993 through 1999. DESCRIPTION OF SYSTEM: The Pregnancy Risk Assessment Monitoring System (PRAMS) is an ongoing, state- and population-based surveillance system designed to monitor selected self-reported maternal behaviors and experiences that occur before, during, and after pregnancy among women who deliver a live-born infant. PRAMS employs a mixed-mode data collection methodology; up to three self-administered surveys are mailed to a sample of mothers, and nonresponders are followed up with a telephone interview. Self-reported survey data are linked to selected birth certificate data and weighted for sample design, nonresponse, and noncoverage to create annual PRAMS analysis data sets. PRAMS generates statewide estimates of various perinatal health topics among women delivering a live infant. Data for 1999 from 17 states are examined. In addition, trend data are examined for 12 states that had at least 3 years of data during 1993-1999. RESULTS: In 1999, the prevalence of unintended pregnancy resulting in a live birth ranged from 33.7% to 52% across the 17 states. During 1993-1999, only one state reported a decreasing trend in the prevalence of unintended pregnancy. Women aged <20 years, black women, women with less than or equal to a high school education, and women receiving Medicaid were more likely to report unintended pregnancy. The prevalence of late or no entry into prenatal care ranged from 16.1% to 29.9%. The prevalence of late or no entry into prenatal care significantly decreased over time in seven of the 12 states with trend data. In general, women aged <20 years, black women, women with less than a high school education, and women receiving Medicaid were more likely to report late or no entry into prenatal care. The prevalence of smoking during the last 3 months of pregnancy ranged from 6.2% to 27.2%, and the prevalence decreased in five states from 1993 to 1999. Overall, smoking during the last 3 months of pregnancy was associated with younger age (<25 years), non-Hispanic ethnicity, having less than or equal to a high school education, receiving Medicaid, and delivering a low birthweight infant. The prevalence of physical abuse by a husband or partner during pregnancy ranged from 2.1% to 6.3%. No trends were observed for physical abuse from 1996 to 1999, the only years for which these data were available. Across the 17 states, only Medicaid status was consistently associated with experiencing physical abuse during pregnancy. The prevalence of breast-feeding initiation ranged from 48% to 89%. Ten of 12 states with trend data reported increases in the prevalence of breast-feeding initiation. Overall, women aged <20 years, women with less than or equal to a high school education, and women receiving Medicaid were less likely to breast-feed. The prevalence of breast-feeding duration for at least 4 weeks ranged from 34.9% to 78.1%. From 1993 to 1999, increases in levels of breast-feeding for at least 4 weeks were observed in eight states. Women aged <25 years, black women, women with less than or equal to a high school education, and women receiving Medicaid were generally less likely to breast-feed for at least 4 weeks. The prevalence of back sleep position for infants ranged from 35.1% to 74.6%. Increases in the use of the back sleep position were observed in all 12 states with trend data from 1996 to 1999. Black race and having less than or equal to a high school education were consistently associated with not using the back sleep position. INTERPRETATION: For surveillance during 1993-1999, the majority or all states observed increases in breast-feeding initiation, breast-feeding for at least 4 weeks, and back sleep position. Approximately one half of the states observed decreases for late or no entry into prenatal care and smoking during the last 3 months of pregnancy. Little or no progress was observed in the prevalence of unintended pregnancy or physical abuse during pregnancy. With few exceptions, the 17 states failed to meet the Healthy People 2000 objectives for the seven reported behaviors in 1999. Certain de

Adolescent↗
PubMed2006

Objective criteria to assess representativity of soil fungal community profiles.

Soil fungal community structures are often highly heterogeneous even among samples taken from small field plots. Sample pooling is widely used in order to overcome this heterogeneity, however, no objective criteria have yet been defined on how to determine the number of samples to be pooled for representatively profiling a field plot. In the present study PCR/RFLP and T-RFLP analysis of fungal 18S rDNA in ten soil samples obtained from a grassland plot of 400 m(2) also revealed this known heterogeneity in fungal community structures. Based on these data a three-step approach to assess representativity of fungal community profiles was established. First, soil DNA quantities needed for robust community profiling were determined. Second, profiles of single or multiple samples were theoretically averaged to test for statistically significant clustering in order to determine the minimal number of samples to be pooled to achieve representativity. Third, DNA extracts of single or multiple samples were pooled prior to profiling in order to test for statistically significant clustering. Analyses revealed robust profiles for 50 ng soil DNA but not for 5 ng. Averaged T-RFLP profiles from five or more soil samples and experimental T-RFLP profiles from pools of seven or more samples formed one significant branch. Theoretical averaging and experimental pooling revealed that five to seven samples have to be pooled for robustly representing the field plot. Our results demonstrate that representativity of soil fungal community profiles can objectively be determined for a field plot with only little deviation between theoretical and experimental approaches. This three-step approach will be of assistance for designing sampling and pooling strategies for comparative analyses of soil fungal communities in ecological studies.

Cluster Analysis↗
PubMed2004

Parents' beliefs and practices regarding childhood fever: a study of a multiethnic and socioeconomically diverse sample of parents.

OBJECTIVE: To examine childhood fever beliefs and practices in a multiethnic, multiracial, and socioeconomically diverse sample. DESIGN: Cross-sectional survey. PARTICIPANTS: Parents of children attending a hospital clinic, neighborhood health center, and a private practice. RESULTS: Two hundred seventy-four parents were interviewed; 45% were Latino; 15%, African-American; 25%, white; and 14%, other (excluded from further analysis, leaving a final sample size of 235). Latinos (11%) and African-Americans (7%) were more likely than whites (0%) to have no thermometers (P < 0.006). Only 42% of parents knew the correct temperature for fever. Greater proportions of Latinos (55%) and African-Americans (44%) take their child to the emergency department for fever than whites (21%; P < 0.001 and P = 0.02, respectively). Multivariate analyses revealed that parents who had not graduated from high school had 5 times the odds of not using a thermometer to check for fever and triple the odds of not asking a health care provider for fever advice, and parents of uninsured children were 5 times less likely to bring their febrile child to the emergency department. Low-income parents significantly more often used the emergency department and remedies other than antipyretics for their febrile children. Latino parents were more likely to believe that certain Latino folk illnesses cause fever, and African-American parents had triple the odds of not knowing the correct temperature for fever. CONCLUSIONS: Parents' beliefs and practices regarding childhood fever vary by race, ethnicity, sociodemographics, and the child's insurance coverage. Educating parents about fever, improving access to health insurance and primary care, and ensuring that families have thermometers may enhance appropriate use of health services and improve outcomes for febrile children.

Adolescent↗
PubMed2006

An empirical verification of population assignment methods by marking and parentage data: hatchery and wild steelhead (Oncorhynchus mykiss) in Forks Creek, Washington, USA.

Assignment tests are increasingly applied in ecology and conservation, although empirical comparisons of methods are still rare or are restricted to few of the available approaches. Furthermore, the performance of assignment tests in cases with low population differentiation, violations of Hardy-Weinberg equilibrium and unbalanced sampling designs has not been verified. The release of adult hatchery steelhead to spawn in Forks Creek in 1996 and 1997 provided an opportunity to compare the power of different assignment methods to distinguish their offspring from those of sympatric wild steelhead. We compared standard assignment methods requiring baseline samples (frequency, distance and Bayesian) and clustering approaches with and without baseline information, using six freely available computer programs. Assignments were verified by parentage data obtained for a subset of returning offspring. All methods provided similar assignment success, despite low differentiation between wild and hatchery fish (F(ST) = 0.02). Bayesian approaches with baseline data performed best, whereas the results of clustering methods were variable and depended on the samples included in the analysis and the availability of baseline information. Removal of a locus with null alleles and equalizing sample sizes had little effect on assignments. Our results demonstrate the robustness of most assignment tests to low differentiation and violations of assumptions, as well as their utility for ecological studies that require correct classification of different groups.

Alleles↗
PubMed1995

[Surveillance of the nutritional status of the population in Kinshasa, Zaire (1991-1994)].

Six anthropometric surveys using a random cluster sampling design have been organised in the capital of Zaire between November 1991 and March 1994. The prevalence of acute malnutrition fluctuated with the seasons and followed the cyclic movements of food prices on the market. The general tendency was rising. In March '94, 10.7% (95% confidence interval: 8.8-12.9%) of the children was suffering of acute malnutrition. Nevertheless, the standardised prevalence remained stable. This may suggest that the most disadvantaged living at certain poor zones of the city disconnected, while the rest of the population could still manage to maintain their nutritional status. The hypothesis of increasing heterogeneity is supported by the raising of the design-effect.

Child, Preschool↗
PubMed2002

Towards a long-term integrated monitoring programme in Europe: network design in theory and practice.

Long-term integrated monitoring is an important approach for investigating, detecting and predicing the effects of environmental changes. Currently. European freshwaters, glaciers, forests and other natural and semi-natural ecosystems and habitats are monitored by a number of networks established by different organisations. However, many monitoring programmes have a narrow focus (e.g. targeting individual ecosystems) and most have different measurement protocols and sampling design. This has resulted in poor integration of ecosystem monitoring at a European level, leading to some overlapping of efforts and a lack of harmonised data to inform policy decisions. The need for a consistent pan-European long-term integrated monitoring of terrestrial systems programme is recognised in the scientific community. However, the design of such a system can be problematic, not least because of the constraints imposed by the need to make maximum use of existing sites and networks. Based on the outcomes of the NoLIMITS project (Networking of Long-term Integrated Monitoring in Terrestrial Systems). this article reviews issues that should be addressed in designing a programme based on existing monitoring sites and networks. Four major design issues are considered: (i) users' requirements, (ii) the need to address multiple objectives, (iii) role of existing sites and (iv) operational aspects.

Altitude↗
PubMed2005

Availability of accessible publications: designing a methodology to provide reliable estimates for the Right to Read Alliance.

OBJECTIVES: To establish reliable estimates of the proportion of books available in formats accessible to visually impaired people; to recommend a practical method of updating the estimates. METHODS: The project had two stages. Stage one estimated the overall availability of all categories of books. The British National Bibliography (BNB) for 1999-2003 was chosen as the sampling frame. A sample of 2069 titles over the 5 years was selected using a systematic sample design. These titles were then checked for availability against the catalogues of four major producer/suppliers of materials in accessible formats. The second stage involved checking the availability of books in six genres; about 400 titles for each genre were examined. RESULTS: Of the titles selected for the overall estimate, 92 (4.4%) were found to be available in one or more accessible formats. Availability of titles in accessible formats varied greatly between categories. Adult fiction had the highest number of available publications (26.3%), least represented were cookery (1.9%) and gardening (1.8%). CONCLUSIONS: This study provided baseline figures and a methodology for the Right to Read Alliance to monitor their progress in campaigning for more books to be accessible to visually impaired people.

Blindness↗
PubMed1993

Determination of phloroglucinol in human plasma by gas chromatography-mass spectrometry.

A specific and sensitive method has been developed for the determination of phloroglucinol in plasma; it involves an optimized procedure for blood sampling designed to minimize the in vitro oxidation of the molecule, and gas chromatography-mass spectrometry after silylation of the compound. The method allowed a reliable determination of phloroglucinol in plasma. The precision and accuracy of the assay, reported as coefficients of variation, were below 15%. Using a plasma sample of 0.25 ml, the limit of quantitation was 5 ng/ml with a precision of 17.4%, which is sensitive enough for pharmacokinetic studies. Stability studies under different conditions revealed that ascorbic acid limits the degradation of phloroglucinol in plasma during storage at freezer temperatures.

Drug Stability↗
PubMed2004

Prevalence of "growing pains" in young children.

We estimated the prevalence of recurrent leg pains, often described as "growing pains" in children 4 to 6 years of age in South Australia. We used a survey of the parents of children 4 to 6 years of age, using a validated questionnaire previously developed for this purpose. The sample was systematic and randomized across rural and urban regions, with a total of 1445 valid responses achieved. Frequency statistics were used to obtain the prevalence estimate. The prevalence estimate obtained was 36.9% (95% CI, 32.7-41.1). This study estimated the prevalence of growing pains in a well-designed sample by using a validated instrument of measure. Previous studies have not addressed this age range discretely. The prevalence estimate demonstrates the community impact of this often disregarded condition.

Child↗
PubMed2006

The nature and fate of natural resins in the geosphere. XII. Investigation of C-ring aromatic diterpenoids in Raritan amber by pyrolysis-GC-matrix isolation FTIR-MS.

Upper Cretaceous amber from the Raritan Formation (Sayerville, New Jersey) has been investigated by Pyrolysis-GC-MS and Pyrolysis-GC-matrix isolation FTIR-MS. Results establish the existence of two distinct forms of amber in this deposit. Both forms are Class Ib ambers, but they are unambiguously differentiated on the basis of their (intact) diterpenoid composition. The presence of callitrisate in both forms, and cupraene in samples designated form 1, strongly suggest that both derive from related-but-distinct species within the Cupressaceae. In addition to callitrisate, dehydroabietate and analogous 17-nor-, 16,17-dinor- and 15,16,17-trinor-analogues of these compounds are also observed. The distributions of these products in multiple samples suggest that they are the result of biological emplacement, rather than diagenetic modification of the parent compounds. This indicates that the distributions of diterpenes observed in these samples are representative of the original bioterpenoids and, hence, are useful for chemotaxonomic analyses.

Journal Article↗
PubMed2000

[Development and design of a sampling kit for microbiological studies].

The paper deals with the development and design of a kit for choosing samples from various objects for microbiological studies. The studies clarified the composition of the set and requirements for the complete set and its parts. The design of this set parts was made and the design documents were drawn up for manufacturing the sampling set. A programme and procedures for microbiological and pilot tests of the kit were developed and its trials were performed.

Drug Design↗
PubMed2006

Scoring system for the selection of high-risk patients in the intensive care unit.

OBJECTIVE: Patients admitted to the intensive care unit greatly differ in severity and intensity of care. We devised a system for selecting high-risk patients that reduces bias by excluding low-risk patients and patients with an early death irrespective of the treatment. DESIGN: A posteriori analysis of a multiple-center prospective observational trial. SETTING: A total of 89 units from 12 European countries, with 12,615 patients. INTERVENTION: Demographic and clinical data: severity of illness at admission, daily score of nursing workload, length of stay, and hospital mortality. METHODS: We enrolled patients with intensive care unit length of stay of >24 hrs. Three groups of high-risk patients were created: a) Severity group, those with Simplified Acute Physiology Score (SAPS II) over the median; b) Intensity-of-care group, patients with >1 day of high level of care (assessed by logistic analysis); and c) MIX group, patients fulfilling both Severity and Intensity-of-care criteria. The groups were included in a logistic regression model (random split-sample design) to identify the characteristics associated with hospital mortality. We compared the outcome prediction of the SAPS II model (unsplit sample) against our model. MAIN RESULTS: Out of 8,248 patients, the Severity method selected 3,838 patients, Intensity-of-care selected 4,244, and both methods combined selected 2,662 patients. There were 2,828 low-risk patients. Significant associations with hospital mortality were observed for: age, sites of admission, medical/unscheduled surgical admission, acute physiologic score of SAPS II, and the indicator variable "only Severity," "only Intensity-of-care," or MIX (developmental sample: calibration chi-square test, p = .205; area under the receiver operation characteristic curve, 0.814). Calibration and discrimination were better in our model than with the SAPS II model (unsplit sample). CONCLUSION: All three indicator variables select high-risk patients, the Severity/Intensity-of-care MIX being the most robust. These stratification criteria can improve case-mix selection for clinical and organizational studies.

Aged↗
PubMed2005

Violent behavior by girls reporting violent victimization: a prospective study.

OBJECTIVE: To assess the relationships between individual victimization and neighborhood-level violence on subsequent violent perpetration by adolescent girls in a community-based sample. DESIGN: Longitudinal, multilevel analysis of data collected by the Project on Human Development in Chicago Neighborhoods. Three in-home interviews were conducted approximately 24 months apart between November 1995 and January 2002 with youth and their caregivers. Community-level data also were collected in 1995 from a random sample of Chicago residents. Hierarchical regression models and propensity scores were used. SETTING: Families and neighborhoods in Chicago. PARTICIPANTS: Population-based sample of 637 girls, ages 9 to 15 years at baseline, and the neighborhoods in which they resided. This sample is diverse in race/ethnicity, socioeconomic status, family structure, and neighborhood characteristics. MAIN OUTCOME MEASURE: Self-reports of violent behavior in the 12 months before the third interview. RESULTS: At baseline, 38% of the girls reported perpetrating at least 1 violent behavior in the prior 12 months, 28% reported past year violent behavior at the first follow-up interview, and 14% reported past year violent behavior at the third interview. The odds of violent behavior were 2.2 times higher among girls who reported prior violent victimization, after prior confounding factors and baseline violent behavior were controlled (95% confidence interval, 1.3- 4.4). Homicides and concentrated poverty in girls' neighborhoods also were associated with aggression by girls. CONCLUSIONS: Improving safety in communities and homes may reduce rates of violent perpetration by adolescent girls. Study results suggest that, to facilitate identification of and healing among adolescent survivors of violence, practitioners should recognize perpetration of violence as potential sequelae of prior violent victimization.

Adolescent↗
PubMed1995

Sib-based detection of QTLs.

Association and transmission/nontransmission analyses were used in a split sample design to identify disease susceptibility alleles at two loci. Sib-pair analysis on various subsets of the data identified an additional four regions that yielded signals of disease predisposing quantitative trait loci (QTLs). Three of these four regions represented Type I errors. A new simulation indicates that a multiplex sampling strategy would substantially improve QTL detection for this oligogenic transmission model.

Alleles↗
PubMed2006

Variation in the gene encoding the serotonin 2A receptor is associated with outcome of antidepressant treatment.

Depressive disorders account for a large and increasing global burden of disease. Although the condition of many patients improves with medication, only a minority experience full remission, and patients whose condition responds to one medication may not have a response to others. Individual variation in antidepressant treatment outcome is, at present, unpredictable but may have a partial genetic basis. We searched for genetic predictors of treatment outcome in 1,953 patients with major depressive disorder who were treated with the antidepressant citalopram in the Sequenced Treatment Alternatives for Depression (STAR*D) study and were prospectively assessed. In a split-sample design, a selection of 68 candidate genes was genotyped, with 768 single-nucleotide-polymorphism markers chosen to detect common genetic variation. We detected significant and reproducible association between treatment outcome and a marker in HTR2A (P range 1 x 10(-6) to 3.7 x 10(-5) in the total sample). Other markers in HTR2A also showed evidence of association with treatment outcome in the total sample. HTR2A encodes the serotonin 2A receptor, which is downregulated by citalopram. Participants who were homozygous for the A allele had an 18% reduction in absolute risk of having no response to treatment, compared with those homozygous for the other allele. The A allele was over six times more frequent in white than in black participants, and treatment was less effective among black participants. The A allele may contribute to racial differences in outcomes of antidepressant treatment. Taken together with prior neurobiological findings, these new genetic data make a compelling case for a key role of HTR2A in the mechanism of antidepressant action.

Adult↗
PubMed2003

Human population structure and its effects on sampling Y chromosome sequence variation.

The excess of rare variants in global sequencing studies of the nonrecombining portion of the Y chromosome (NRY) has been interpreted as evidence for the effects of human demographic expansion. However, many NRY polymorphisms are geographically localized and the effect of different geographical sampling on patterns of NRY variation is unknown. We use two sampling designs to detect population structure and its effects on patterns of human NRY polymorphism. First, we sequence 26.5 kb of noncoding Y chromosome DNA from 92 globally distributed males representing 35 populations. We find that the number of polymorphisms with singleton variants is positively correlated with the number of populations sampled and that there is a significant negative correlation of Tajima's D (TD) and Fu and Li's D (FD) statistics with the number of pooled populations. We then sequence the same region in a total of 73 males sampled from 3 distinct populations and find that TD and FD values for the 3 pooled and individual population samples were much less negative than those in the aforementioned global sample. Coalescent simulations show that a simple splitting model of population structure, with no changes in population size, is sufficient to produce the negative values of TD seen in our pooled samples. These empirical and simulation results suggest that observed levels of NRY population structure may lead to an upward bias in the number of singleton variants in global surveys and call into question inferences of population expansion based on global sampling strategies.

Asian People↗