Population Centroid Research Articles

Geographic Access to Health Care for Rural Medicare Beneficiaries

Patients in rural areas may use less medical care than those living in urban areas. This could be due to differences in travel distance and time and a utilization of a different mix of generalists and specialists for their care. To compare the travel times, distances, and physician specialty mix of all Medicare patients living in Alaska, Idaho, North Carolina, South Carolina, and Washington. Retrospective design, using 1998 Medicare billing data. Travel time was determined by computing the road distance between 2 population centroids: the patient's and the provider's zone improvement plan codes. There were 2,220,841 patients and 39,780 providers in the cohort, including 6,405 (16.1%) generalists, 24,772 (62.3%) specialists, and 8,603 (21.6%) nonphysician providers. There were 20,693,828 patient visits during the study. The median overall 1-way travel distance and time was 7.7 miles (interquartile range 1.9-18.7 miles) and 11.7 minutes (interquartile range 3.0-25.7 minutes). The patients in rural areas needed to travel 2 to 3 times farther to see medical and surgical specialists than those living in urban areas. Rural residents with heart disease, cancer, depression, or needing complex cardiac procedures or cancer treatment traveled the farthest. Increasing rurality was also related to decreased visits to specialists and an increasing reliance on generalists. Residents of rural areas have increased travel distance and time compared to their urban counterparts. This is particularly true for rural residents with specific diagnoses or those undergoing specific procedures. Our results suggest that most rural residents do not rely on urban areas for much of their care.

Geographically Based Investigation of Prostate Cancer Mortality in Four U.S. Northern Plain States

Historically, prostate cancer mortality rates have been elevated in the U.S. Northern Plains states. The purpose of this study was to investigate possible contributing factors, especially whether there was any association with crop patterns. Prostate cancer mortality rates (1950-2000) in four northern plains states (MN, MT, ND, and SD) were compared to rates for 46 other U.S. states. Within the four states, county rates in urban, less urban, and rural areas also were compared. For additional analysis, urban counties and counties with <10% of county area in crops were excluded. The average percent of county area in total cropland 1930-1950 and 1954-1974 was estimated. Using Poisson regression, we investigated whether the average percentage of county area in total cropland, 1930-1950 and 1954-1974, was associated with prostate cancer mortality rates, 1975-2000, respectively. Poisson regression analyses were also used to evaluate associations between rates and major crops, which included spring and durum wheat, winter wheat, corn, and other crops. Population centroids of the Census 2000 block groups were used to estimate the percentage of males aged 35 and older residing in close proximity to small grains crops. Mortality rates were higher in rural compared to urban counties in 1950-2000 (rate ratio [RR]=1.032; 95% CI=1.001-1.063). Rates in 1950-1974 were significantly associated with production of corn and other crops in 1930-1950 (corn: RR per 10% increase=1.033, 95% CI=1.012-1.054; other crops: RR=1.042, 95% CI=1.021-1.063). Mortality rates in 1975-2000 were significantly associated with spring and durum wheat production in 1954-1974 (RR per 10% increase=1.042, 95% CI=1.017-1.067). Prostate cancer mortality rates increased as the percentage of population living within 500 m of small grains crops increased. Epidemiologic studies to evaluate agricultural practices are warranted to further evaluate the observed associations.

A comparison of methods for calculating population exposure estimates of daily weather for health research

BackgroundTo explain the possible effects of exposure to weather conditions on population health outcomes, weather data need to be calculated at a level in space and time that is appropriate for the health data. There are various ways of estimating exposure values from raw data collected at weather stations but the rationale for using one technique rather than another; the significance of the difference in the values obtained; and the effect these have on a research question are factors often not explicitly considered. In this study we compare different techniques for allocating weather data observations to small geographical areas and different options for weighting averages of these observations when calculating estimates of daily precipitation and temperature for Australian Postal Areas. Options that weight observations based on distance from population centroids and population size are more computationally intensive but give estimates that conceptually are more closely related to the experience of the population.ResultsOptions based on values derived from sites internal to postal areas, or from nearest neighbour sites – that is, using proximity polygons around weather stations intersected with postal areas – tended to include fewer stations' observations in their estimates, and missing values were common. Options based on observations from stations within 50 kilometres radius of centroids and weighting of data by distance from centroids gave more complete estimates. Using the geographic centroid of the postal area gave estimates that differed slightly from the population weighted centroids and the population weighted average of sub-unit estimates.ConclusionTo calculate daily weather exposure values for analysis of health outcome data for small areas, the use of data from weather stations internal to the area only, or from neighbouring weather stations (allocated by the use of proximity polygons), is too limited. The most appropriate method conceptually is the use of weather data from sites within 50 kilometres radius of the area weighted to population centres, but a simpler acceptable option is to weight to the geographic centroid.

The effect of alternative representations of population location on the areal interpolation of air pollution exposure

Relatively new accurate commercial point databases incorporating population distribution are now available which could enhance areal interpolation estimates required for an underlying population. This paper explores the level to which results will be dependent on how well the underlying population is represented. Using a number of different levels of detail to represent the population distribution (postcode units, enumeration districts (ED) population centroids and no population information), this paper shows the differences that can occur in computing average ED dose levels due to the degree to which the population is represented. Although generally the differing methodologies gave similar overall patterns, there were substantial disparities between specific ED results obtained. As anticipated, the sensitivity of results were influenced by the degree to which the method's assumptions were adhered to. In the case of the ED population centroid approach, the extent to which the summarising points reflect the underlying spatial objects is critical to the estimated result. For areal weighting, the results were significantly influenced by the level of homogeneity within the variable of interest (population distribution). Generally, greater variation within the pollution surface exacerbates any breach in the methods underlying assumptions.

Stroke Mortality Associated With Living Near Main Roads in England and Wales

Air pollution is associated with stroke, and road traffic is a major source of outdoor air pollution. Using proximity to roads as a proxy for exposure to road traffic pollution, we examined the hypothesis that living near main roads increases the risk of stroke mortality. We used a small-area ecological study design based on 113 465 census enumeration districts in England and Wales. Stroke mortality (International Classification of Disease, 9th revision, codes 430 through 438) in England and Wales from 1990 to 1992 for people >or=45 years of age was examined through the use of 1991 population denominators. Exposure was calculated as distance from each enumeration district population centroid to the nearest main road. We adjusted for age, sex, socioeconomic deprivation (using Carstairs index), regional variation, urbanization, and metropolitan area using Poisson regression. The analysis was based on 189 966 stroke deaths and a population of 19 083 979. After adjustment for potential confounders, stroke mortality was 7% (95% confidence interval [CI], 4 to 9) higher in men living within 200 m of a main road compared with men living >or=1000 m away. The corresponding increase in risk for women was 4% (95% CI, 2 to 6) and the risk for men and women combined was 5% (95% CI, 4 to 7). These raised risks diminished with increasing distance from main roads. Living near main roads is associated with excess risk of mortality from stroke, and if causality were assumed, approximately 990 stroke deaths per year would have been attributable to road traffic pollution.

Tests of the Las Campanas Distant Cluster Survey from Confirmation Observations for the ESO Distant Cluster Survey

The ESO Distant Cluster Survey (EDisCS) is a photometric and spectroscopic study of the galaxy cluster population at two epochs, z~0.5 and z~0.8, drawn from the Las Campanas Distant Cluster Survey (LCDCS). We report results from the initial candidate confirmation stage of the program and use these results to probe the properties of the LCDCS. Of the 30 candidates targeted, we find statistically significant overdensities of red galaxies near 28. Of the ten additional candidates serendipitously observed within the fields of the targeted 30, we detect red galaxy overdensities near six. We test the robustness of the published LCDCS estimated redshifts to misidentification of the brighest cluster galaxy (BCG) in the survey data, and measure the spatial alignment of the published cluster coordinates, the peak red galaxy overdensity, and the brightest cluster galaxy. We conclude that for LCDCS clusters out to z~0.8, 1) the LCDCS coordinates agree with the centroid of the red galaxy overdensity to within 25'' (~150 h^{-1} kpc) for 34 out of 37 candidates with 3\sigma galaxy overdensities, 2) BCGs are typically coincident with the centroid of the red galaxy population to within a projected separation of 200 h^{-1} kpc (32 out of 34 confirmed candidates), 3) the red galaxy population is strongly concentrated, and 4) the misidentification of the BCG in the LCDCS causes a redshift error >0.1 in 15-20% of the LCDCS candidates. These findings together help explain the success of the surface brightness fluctuations detection method.

Genetic drift and gene flow in a prehistoric population of the Azapa valley and coast, Chile.

The present paper studies the evolutionary process operating on prehistoric groups from the Azapa valley and coast (northern Chile). The sample consists of 237 crania from the Archaic Late, Early Intermediate, Middle, Late Intermediate, and Late periods. Six metric variables were used, which were transformed to eliminate the special environmental component and to increase the proportion of genetic variance. Population structure was assessed using a method based on quantitative genetic theory, which predicts a lineal relationship between average within-group phenotypic variance and group distance to the population centroid. Results indicate that 17.5% of the total variance accounts for special environmental variance. An excess in extraregional genetic flow is observed in the population corresponding to the Early Intermediate period in the valley. A reduced differentiation is observed between Archaic and Early Intermediate coastal groups, as well as between the latter and the inhabitants of the valley in the same period. Genetic differences between both areas increased substantially since the Middle period. Evidence indicates that long-range gene flow was lower on the coast than in the valley, the lowest estimated Fst being 0.0199 for the total population (coast and valley), 0.0111 for the coastal population, and 0.0057 for the valley. Results are discussed in terms of regional archeological and ethnohistorical evidence, and a microevolutionary model is presented to account for the biological history of the population.

An integrated GIS and modeling approach for assessing the transient response of forests of the southern Great Lakes region to a doubled CO 2 climate

This research examines the effects of climate change on the species composition of forests in the southern Great Lakes region in USA (Illinois, Indiana, and Ohio) by simultaneously addressing five key components necessary for realistic predictions of future forest composition. We simulated transient (1), species-level (2), forest response to climate change at a spatial scale that accounted for competitive effects (3), and regional site diversity (4), in the spatial configuration of forests within the regional landuse matrix (5). The JABOWA-II forest growth model was used to provide species-specific responses of 45 tree species to site conditions (e.g. climatic, edaphic) while accounting for competition for limited resources (e.g. light, nutrients, water). Forest growth was simulated annually from 1981 to 2060 in 0.01 ha plots, each representing a unique 4 km 2 area (site) in the region with 30% or more forest cover (16,431 unique sites). Geo-spatial input data included land use (Land Use/Land Cover Data), community-level tree species composition (Eastwide Forest Inventory Database), site conditions (State Soils Geographic Data Base, Digital Elevation Model), baseline climate (1951–1980 normals from National Oceanic and Atmospheric Administration weather stations), and general circulation model output for generating the changed climate scenario (2×CO 2 equilibrium run, Oregon State University model). The equilibrium run output, which predicted temperature increases ranging from 3.11 to 3.67 °C and precipitation increases from 2 to 14% over the study area, was applied linearly to the baseline climate over an 80-year modeling period. Results from forty iterations of simulated forest growth under each climate scenario (baseline and changed) were compiled at 10-year intervals as total basal area for each species. These plot results were scaled to the appropriate site according to the amount of forest cover in the 4 km 2 areas. Dominance-weighted population centroids were calculated for the regional distribution of each species for both climate scenarios at the years 1981 and 2060. Relative to forest conditions at 2060 after growth under baseline climate, the climate change scenario caused a decrease in total basal area of Northern Conifers (−99.9%) and Northern Deciduous (−99.4%) species and only a slight increase in Intermediate (+1.23%) and Southern species (+0.3%). Direction of population centroid shifts for these four species groups primarily were northeast. Our results for the northern species groups were consistent with previous studies. Possibly, our most important results were the minor differences in relative response for Intermediate and Southern species at the group level.

Morphometric variation in red-spot prawn ( Metapenaeopsis barbata) in different geographic waters off Taiwan

Morphometric variation was used to elucidate the stock structure of red-spot prawn ( Metapenaeopsis barbata) in adjacent waters off Taiwan. Five samples were separately collected from Keelung, Tashi, Taichung, Putai, and Cheding. The 10 measurements made for each individual were size-standardized by Burnaby’s method and resulting measurements submitted to canonical variate analysis; the square of Manhattan distance (Manhattan D 2) between population centroids values were then calculated using canonical scores. Dendrograms were constructed by three different clustering methods using Manhattan D 2 values. A randomization test was used to test the significance of morphometric variation between different groups derived from cluster analysis. A sexual dimorphism for red-spot prawn was found by ANCOVA, so analyses were separately performed by sex. Five sampling areas were clustered into two groups by each sex, the first group included Keelung; the second group included Tashi, Taichung, Putai, and Cheding. Randomization tests showed that morphometric variation between these two groups was significant for each sex. There appear to be at least two morphologically distinguishable stocks of red-spot prawn in adjacent waters off Taiwan. The utility of morphometric measurements for discriminating stock of red-spot prawn is demonstrated, but further verification of the stock structure may be essential.

Migration Drift

For perhaps the first time since the founding of the United States the net direction of interstate migration was to the east rather than the west for the period 1992–1993 through 1994–1995. This and other findings, such as a general slowdown in the southward tendency of U.S. population movement over the period 1980–1981 to 1994–1995, are highlighted using the concept of migration drift. I propose the migration drift measure to summarize the net directionality and distance moved by migrants in any country's population system. Like the concept of a center of population or a population centroid, migration drift is an intuitive one. Unlike the population centroid, however, migration drift summarizes the pure effects of population movement without confounding those influences with spatial variations in birth, death, and net foreign immigration rates.

Morphological variation in sexual and agamospermous Amelanchier (Rosaceae)

Agamospermy, which is almost always associated with polyploidy, is often assumed to reduce variation and foster evolution of microspecies. We tested for the occurrence of microspecies by comparing variation of sexual Amelanchier bartramiana and facultatively agamospermous (asexually seed-producing) Amelanchier laevis. We assessed within- and among-population variation of 222 individuals from six Maine populations of each species for eight morphological variables. Mahalanobis distances between individuals and population centroids and between population centroids and species centroids were used as measures of within- and among-population variation, respectively. Amelanchier bartramiana contains significantly more within- and among-population morphological variation than A. laevis. The two species do not differ in how they partition morphological variation within and among populations. Amelanchier laevis thus does not contain microspecies. Variation within A. laevis may be the result of sexuality, hybridization, polyploidy, and other factors. Key words: Amelanchier, agamospermy, Levene's statistic, microspecies, variation.

Morphological Variation between Geographically Disjunct Populations of Atlantic Salmon: The Effects of Ontogeny and Habitat Shift

Summary 1. Previous studies suggest that morphological divergence among populations is a widespread result of reproductive isolation and local adaptation. In this paper I test the hypothesis that morphological differentiation should be reduced when individuals shift from a geographically heterogeneous habitat (benthic, fresh water) to a relatively uniform habitat (pelagic, open ocean). 2. Juvenile Atlantic Salmon Salmo salar from two disjunct populations but reared under the same environmental conditions differed in a number of morphometric traits and general body shape in both the freshwater and the premigratory stage associated with the oceanic phase. 3. Relative to the premigratory phase, the riverine forms of these populations differed in a larger number of variables, the discriminant functions correctly assigned a larger proportion of individuals to their source population and morphological distances between population centroids were also greater. 4. Morphometric distances between the centroids of stages within populations were greater than distances between populations within stages. 5. Though preliminary, these results suggest that, for some traits, isolated populations may diverge in a determined stage but converge in another, especially in species that undergo a habitat shift.

Biology of aging in an Israeli population. 2. Polymorphic blood markers and fluctuating asymmetry

The major objectives of the present study were to detect whether there are any differences in genetic blood markers distribution between the young and old Israeli population. We also investigated the question about the relationship between the genetic heterozygosity (H) and fluctuating asymmetry (FA) of an individual. The study sample consisted of 208 elderly people aged between 75 and 94 years old. Each individual was assessed for the same 18 anthropological traits, 8 of which were paired bilateral structures and 10 were size and mass measurements. Thirteen polymorphic gene markers were also taken from each individual. Most of the studied gene systems showed no inter-group (old sample vs 207 young individuals) differences. Significant differences were detected at ADA, ESD, GTP1 and FY loci. These differences were mainly due for the considerably lower frequency of the heterozygous individuals in elderly samples. Our working hypothesis at the next stage of the analysis were as follows: 1. Morphologically central phenotypes are more symmetric (processes lower FA) than morphologically extreme individuals. 2. There is a detectable negative correlation between the FA and individuals H. To study these questions we developed a multivariate measure of FA and morphological deviation of the individual from the population centroid. Yet, no reliable evidence in support of either of the two hypotheses were obtained.

Relationship between fluctuating asymmetry, morphological modality and heterozygosity in an elderly Israeli population

We have been checking the following working hypotheses: 1) There is a negative correlation between genetic heterozygosity and fluctuating asymmetry (FA); 2) FA is a measure of developmental stability/instability of the whole organism, i.e. we expect negative correlation between FA and morphological proximity of a set of mass-size variables of an individual to a population centroid; and 3) FA is a measure of character-specific stability in a population, i.e. we expect correlation between magnitude of FA and deviation of an individual from the population centroid of the bilateral characters themselves. For this purpose each individual in a sample of about 200 elderly individuals was assessed for 11 polymorphic blood systems (14 genetic loci) as well as for a set of 26 anthropometric traits: 1) a set of ten mass size variables; and 2) a set of eight pairs of bilateral measurements. Four multivariate measures of morphological centrality were computed, two measures for size and two measures of shape distances from the ith individual to the population centroid for mass-size variables and also for the bilateral variables. A multivariate measure of FA for 8 bilateral pairs was also computed. No relationship was detected between FA and heterozygosity, or between FA and any of the four multivariate deviations. Thus, we concluded that our data do not support the listed hypotheses.

Application of phage typing to the identification of sources of groundwater contamination

Phage typing of Escherichia coli populations was used as a “fingerprinting” tool to identify the source(s) of fecal coliform contamination of a drinking water well, PW- 12. Group discriminate analysis was used to evaluate the data and determine the relative distance of population centroids from the centroid of the PW- 12 population. The phage typing patterns were compared to serological results and correlations noted. Phage typing patterns were shown to be stable following 32-days incubation of E. coli isolates in garden soil, tap water and neutralized tertiary effluent, with and without the presence of autochthonous flora.

A view of niche overlap amenable to statistical analysis

Published values of niche overlap (similarity) are generally point estimates of the similarity between population centroids. A number of shortcomings are associated with this method of data presentation: (i) confidence intervals on the estimates are lacking, (ii) no statistical procedures are used to test for significant differences between estimates, and (iii) the estimates tend to be biased. These problems arise primarily as a result of the manner in which data are pooled for the calculations. We illustrate an alternative method of data handling, involving calculation of similarity between individuals, which is more biologically reasonable and which eliminates these problems. A permutation test procedure is also introduced for use on large sets of data.

Morphological variation among widely dispersed larval populations of anadromous southern hemisphere lampreys (Geotriidae and Mordaciidae)

Larvae of all three southern hemisphere anadromous parasitic lampreys were collected from rivers in Australia, New Zealand and South America. Body intervals were measured, trunk myomeres counted and the frequency of pigmentation in different body regions recorded. Morphometric data were subjected to multiple group principal components analysis (MGPCA) which took into account changes during growth. The components (together with myomere counts) and the pigmentation data were both subjected to discriminant analysis. Ordination and rank correlation tests revealed no evidence for either latitudinal clines or a continuum of circumpolar change amongst larval lamprey populations. Clustering of population centroids clearly distinguished between Mordacia lapicida (Gray) from Chile and M. mordax (Richardson) from south-eastern Australia. Populations of Geotria australis Gray divided into groups representing three geographical regions, namely Argentina, Chile and Australasia (Western Australia, Tasmania and New Zealand). Ammocoetes from Argentina were the most divergent, possessing a more posterior cloaca, taller dorsal fins, a greater gap between dorsal fins, and distinctive pigmentation on the head and caudal fin. Within the Australasian group, Western Australian and New Zealand populations clustered closer than either did with those from Tasmania. The cluster analyses for larval populations of G. australis suggest that, during their marine trophic phase, the adults of this species originating from Argentinian and Chilean rivers follow different migratory routes, whereas those from Western Australia, New Zealand and, to a lesser extent, Tasmania intermix.

Discriminant Analysis of Multivariates Taken from a Single Population

The null hypothesis of a one-sample test of multivariate means states that the population centroid is equal to a vector of specified constants. If this hypothesis is rejected, then the distance from the population centroid to the hypothesized centroid is different from zero. The purpose of this paper is to present a linear function (analogous to a discriminant function) which will allow the data analyst to interpret the dimension of the line joining the two centroids. The statistics developed to interpret this dimension are the coefficients of a discriminant function and the correlation of each dependent variable with a discriminant score. A data example is also presented to demonstrate the material and contrast it with conventional methodology.

Inferring History From Crania: Biological Distance Comparisons Of Mill Creek And Early Middle Missouri Tradition Crania With Mandan And Arikara Population Samples

Craniometric data are used to compare available human crania from Mill Creek and early Middle Missouri Tradition sites with a pooled Mandan sample and Coalescent Tradition samples from Crow Creek, Mobridge and hygh (inferred proto-Arikara). Discrim inant functions are computed for the reference samples, and test cases are classified according to their proximity to each population centroid. With one exception, early Middle Missouri Tradition site crania compare most favorably with Mandan. Mill Creek site specimens are unlike the Mandan, most closely resembling early Coalescent samples.

Strong Law of Large Numbers for Measures of Central Tendency and Dispersion of Random Variables in Compact Metric Spaces

Given a sample of independent random variables $Z_1, Z_2, \cdots, Z_n$ with identical distribution $p$ on a compact metric space $(M, d)$, a measure of central tendency is a sample centroid (of order $r > 0$) defined as a point $\hat{X}_n$ in $M$ satisfying $\frac{1}{n} \sum^n_{i=1} d^r(\hat{X}_n, Z_i) = \inf_{x \in M} \frac{1}{n} \sum^n_{i=1} d^r(x, Z_i).$ A (population) centroid of $Z$ is any point $x^\ast$ in $M$ such that $\int_M d^r(x^\ast, z) dp(z) = \inf_{x \in M} \int_M d^r(x, z) dp(z).$ The quantity $\frac{1}{n} \sum^n_{i=1} d^r(\hat{X}_n, Z_i)$ itself is called the sample variation, whereas $\int_M d^r(x^\ast, z) dp(z)$ is the variation of $Z$. This paper establishes almost sure convergence for the sample centroid and variation to the corresponding population values for all orders $r > 0$. Convergence is also proved for the case when the sample centroid is restricted to be one of the sample values.