A model for analyzing insect stage-frequency data when mortality varies with time

Summary A model is developed for the analysis of insect stage-frequency data which may be applied to populations with age-dependent mortality. The analysis of stage-frequency data is divided into two steps. In the first step, the number of different mortality rates and their values are estimated. The second step provides estimates of developmental rates and variances for each developmental stage and in addition provides estimates of the number of recruits to each stage. The model may be used both in analysis and prediction of insect stage frequencies. Hence, in addition to estimating developmental and mortality rates from stage-frequency data, it may also be used as a simulation model for an insect population. The model is applied to two populations ofHemileuca oliviae Cockerell, a lepidopterous pest of New Mexico grasslands. The model identifies, in the two populations, different mortality rates that are related to plant productivity.     

Estimation of the individual number entering each development stage in an insect population

Summary A method for estimating the number entering each development stage from data obtained by regular sampling through one generation of an insect population was described. This method is consisted of the following two procedures: The provisional estimates are calculated on the assumption that each stage has a common mortality in a sampling interval. Then these estimates are corrected on another assumption that the mortality is different in each stage but constant during a stage. The result of testing its validity with two laboratory populations of the common cabbage butterfly,Pieris rapae crucivora, showed the availability of the present method.     

The influence of pheromones produced by freely growing laboratory populations on the reproductive maturation of prairie deermice following prenatal and postnatal exposure

Summary Significantly more females reared on bedding soiled by different populations reproduced by 100 days of age than those reared on bedding soiled by the same populations (P<.1) or on clean bedding (P<.025) whereas the latter two groups did not differ significantly. Significantly greater percentages of females reproduced in both treatments maintained on soiled bedding than was true in the populations from which the bedding came. There were no significant differences between treatments in the weights of the reproductive organs (testes, seminal vesicles, ovaries, uteri) or in the numbers of corpora lutea or mature follicles in the ovaries of nulliparous females. The data suggest that if there were reproductively inhibiting pheromones influencing the population animals, they were altered or dissipated immediately after removal of the bedding from the population enclosures or that they were nonfunctional in the absence of the population animals. These data further suggest that the reproductive inhibition of population young is due to the presence of the populaton animals and that sociobiological factors within the populations act to prevent the reproductively stimulating influence of materials which may be present.     

Note on the accuracy of local population estimates

Mr. Silcock's article will be of interest to all concerned with local population data. It may be useful to supplement it by a brief account of the fuller examination of the local population estimates made in 1951 by the General Register Office, since this covered all 1472 administrative areas in England and Wales and could be made in more detail than was possible for a private investigator.

Any census, of course, provides information not available, at least in such detail, at other times or from other sources, and also serves as a base from which estimates for succeeding years can be derived. In addition, however, the General Register Office takes the opportunity of a census to try and assess the accuracy of the various types of current population estimates made by the Department. In the case of local administrative areas the comparison of actual and expected populations made after the 1931 Census is discussed in the Text Volume of the Registrar General's Statistical Review for 1930 (pages 100-102).     

Infant mortality in the Marshall Islands

Levy and Booth present previously unpublished infant mortality rates for the Marshall Islands. They use an indirect method to estimate infant mortality from the 1973 and 1980 censuses, then apply indirect and direct methods of estimation to data from the Marshall Islands Women's Health Survey of 1985. Comparing the results with estimates of infant mortality obtained from vital registration data enables them to estimate the extent of underregistration of infant deaths. The authors conclude that 1973 census appears to be the most valid information source. Direct estimates from the Women's Health Survey data suggest that infant mortality has increased since 1970-1974, whereas the indirect estimates indicate a decreasing trend in infant mortality rates, converging with the direct estimates in more recent years. In view of increased efforts to improve maternal and child health in the mid-1970s, the decreasing trend is plausible. It is impossible to estimate accurately infant mortality in the Marshall Islands during 1980-1984 from the available data. Estimates based on registration data for 1975-1979 are at least 40% too low. The authors speculate that the estimate of 33 deaths per 1000 live births obtained from registration data for 1984 is 40-50% too low. In round figures, a value of 60 deaths per 1000 may be taken as the final estimate for 1980-1984.     

Estimating African-American mortality from inaccurate data

This paper evaluates the quality of vital statistics and census data for estimating African-American mortality over a period of six decades. The authors employ intercensal cohort comparisons and extinct generation estimates to demonstrate that conventionally constructed African-American death rates may be seriously flawed as early as age 50. Using the crude death rate at ages 50+ for 1978-1982 in conjunction with estimated growth rates and two model life table systems, the authors estimate black age-specific death rates in 1978–1982. These results suggest that if a racial crossover in death rates occurs, the age pattern of mortality among African-Americans must be far outside the range observed in populations with more accurate data.     

Assessing the accuracy of Australia’s small-area population estimates

The Australian Bureau of Statistics provides population estimates of Statistical Local Areas annually. The accuracy of these estimates can be assessed after population estimates are rebased after each quinquennial Census of Population and Housing, however there appears to be no straightforward method of assessing these estimates. Errors that occur with population estimates can be attributed to several factors, both broad and specific to individual areas. These factors include inherent characteristics of the region, such as population size and growth rate; changes in the geographic boundaries; quality of input data; estimation method; and adjustments to control totals (state populations).     

New regression formula to estimate the whole population for recapture-addicted small mammals

Summary We have devised a census formula of curvilinear regression suitable for capture-recapture data of recapture-addicted populations of the Japanese field vole (Microtus montebelli) obtained under a grid-plan with single-catch traps in order to estimate the whole population. The equation is founded on the assumption that the trappable population on the initial day is increased in way of an exponential curve until it reaches to the whole during one trapping period. The effect of trap-preoccupation by marked and multiple collisions is considered in the formula. As a result of its application to field data of the vole, it has turned out that the equation is required for the data gained under the trapping plan with trap spacing 10m, but not for those under the plan with spacing 5m, to estimate the whole. A convenient method of analysis of the formula is offered here, but we have been yet unable to introduce assymptotic variance of estimates. Contributions from JIBP-PT No. 19, carried out by the grant from the expenditure of Education Department to the specific study on “Dynamics of Biosphere”.     

A field study of the effect of prebaiting on censusing by the capture-recapture method in a vole population

Summary Populations of the vole,Clethrionomys rufocanus, in a lowland woodlot of Hokkaido were studied for the presence of effects of prebaiting on censusing by the capture-recapture method. A grid of 121 live-traps, spaced 5 m apart, was laid out on each of two plots, one of which alone was prebaited three days long. Owing to very high densities and great trap-efficiency, sufficient and favourable samples could be available for statistical analysis, except the trend of delayed catch for young in sampling. The population on either plot, however, proved to be markedly variable in catchability of unmarked animals in the course of trapping; while the probability of recapture was counted as invariable on the average from day to day, the recapture frequency was different between juveniles, subadults and adults. Needless to say, the catchability was distinctly greater for marked voles than for unmarked ones, whether prebaited or not, through the trapping period, except that the first-day catchability for unmarked ones on the prebaited plot seems not to be significantly lower than that for marked ones. Consequently, it turns out that the prebaiting has almost never helped to eliminate the important bias induced by differential trap-response of marked and unmarked animals; its contribution is only that the catchability for unmarked ones is slightly higher on the part of the prebaited plot on earlier days of the period. In accordance with the heterogeneous catchability, the Γ-form distribution analogous to the geometric could be applied with thorough fitness to the capture frequency in order to estimate the whole populations. The fact that the estimates are reliable, being not at any rate underestimates, was further confirmed by the result of a follow-up work conducted by means of the removal method with wider trap-spacing which brought forth distinct underestimation chiefly referable to unexposure to traps of the partial populations. The subject of unexposure was discussed by laying stress on the relation between minimum range length and trap-spacing. Contribution from JIBP-PT No. 88, carried out by the grant from the expenditure of Education Department to the specific study on “Dynamics of Biosphere”.     

Demographic Differentials and Demographic Integration of Turkish and Kurdish Populations in Turkey

The objectives of the study are threefold: (1) to provide estimates of the total populations and spatial distributions of different language groups in Turkey, (2) to test whether the commonly held belief that Turkish-speaking and Kurdish-speaking populations are “actors” of different demographic regimes is true, and (3) to assess whether a process of integration, in the form of intermarriage of Turks and Kurds is under way in Turkey. Data come mainly from the 2003 Turkish Demographic and Health Survey (TDHS-2003). Based on the assumption that the mother tongue composition of women is also representative of that of the whole population, the results of the TDHS-2003 imply that of the population of Turkey, 83% are Turkish-speaking, 14% are Kurdish-speaking, 2% are Arabic-speaking and the remaining 1% belong to other language groups. Results show that despite intensive internal migration movements in the last 50 years, strong demographic differentials exist between Turkish and Kurdish-speaking populations, and that the convergence of the two groups does not appear to be a process under way. Turks and Kurds do indeed appear to be actors of different demographic regimes, at different stages of demographic and health transition processes.     

Migration bias in indirect estimates of regional childhood mortality levels

Demographers often use Brass‐style indirect methods to obtain childhood mortality estimates for regions within developing countries. Regional populations are not closed to migration, however, and mortality reports of women resident in a certain region on the survey date may contain information on events and exposure that occurred elsewhere as the mother migrated. Including this “imported”; mortality information may cause significant bias in regional estimates. In this paper the authors: (1) investigate the possible magnitude of migration bias using a multiregional simulation model, (2) propose a modification to standard methods which should reduce bias in many circumstances, and (3) apply the modified technique to data from Brazil's 1980 Census. We find that migration bias can indeed be significant, and that in the specific case of São Paulo state, imported mortality information may result in overestimates of local mortality levels of 10–15% when using Brass‐style methods.     

Bayesian population reconstruction of female populations for less developed and more developed countries

We show that Bayesian population reconstruction, a recent method for estimating past populations by age, works for data of widely varying quality. Bayesian reconstruction simultaneously estimates age-specific population counts, fertility rates, mortality rates, and net international migration flows from fragmentary data, while formally accounting for measurement error. As inputs, Bayesian reconstruction uses initial bias-reduced estimates of standard demographic variables. We reconstruct the female populations of three countries: Laos, a country with little vital registration data where population estimation depends largely on surveys; Sri Lanka, a country with some vital registration data; and New Zealand, a country with a highly developed statistical system and good quality vital registration data. In addition, we extend the method to countries without censuses at regular intervals. We also use it to assess the consistency of results between model life tables and available census data, and hence to compare different model life table systems.     

The Accuracy of Age-specific Population Estimates for Small Areas in Britain

Population estimates play an important role in the allocation of resources at many levels of government and commerce but little is known about the accuracy of age-specific population estimates. Such knowledge is crucial, as resource allocation is often targeted at populations of particular age, and decisions need to be based on the reliability of the estimates. This paper presents a multi-level statistical analysis of the accuracy of age-specific population estimates made for British local authorities in 1991. The aim of this work is to identify the factors that influence accuracy, and to investigate how these influences interact. Our analyses show that the following area characteristics are key factors: true population size; intercensal population change; and percentages of unemployed residents, armed forces residents, and students. In addition, we find that the overall type of method used to calculate estimates is important, and that its effect varies both with area characteristics and with age-group. Local census methods are found to be generally superior, but a low-cost apportionment method, if implemented well, may be as effective.     

Fertility preferences and subsequent childbearing in Africa and Asia: A synthesis of evidence from longitudinal studies in 28 populations

Survey data on fertility preferences have played a central but controversial role in fertility research and advocacy for family planning. We summarize evidence from longitudinal studies in 28 Asian and African populations on the relationship between preferences and subsequent childbearing. While we found no consistent association between women's desire to delay childbearing and subsequent fertility, the baseline desire of women to stop childbearing was a powerful predictor of subsequent fertility in all populations and increased in strength as overall contraceptive use in the study populations rose. Partners’ desire also exercised some influence but was of modest importance in most populations. However, the correspondence between desire to stop and behaviour was found to be far from perfect. Weak implementation of preferences by contraception is likely to be the major cause of this preference–behaviour discrepancy. Uncertainty and instability in preferences may also contribute to the discrepancy, particularly in sub-Saharan Africa.     

A method for the estimation of fecundability

A new method for the estimation of the mean and variance of fecundability is described. The data input required for this procedure is the distribution of the interval from marriage to first birth, or from the resumption of the conception risk after contraction to the subsequent birth. The estimates of the mean and variance of fecundability are obtained by fitting a model to the observed interval distribution. To test the method, it is applied to data from five historical populations. The fecundability means in these populations ranged from 0.18 to 0.31 while the co-efficients of variation all had values near 0.56. A short method for the estimation of the mean of fecundability based on the same model, but not requiring a computer, is also presented.     

Does time preference change with age?

This study looks at compensating differentials in the National Longitudinal Survey of Youth (NLSY) to derive estimates of the levels of time preference for labor force participants in each of 15 waves of data from 1979 to 1994. With these estimates the evolution of time preference over the life course is described. Future utility among labor force participants appears to be valued more highly by subjects who are older, more schooled, white, or male. Controlling for schooling level, a higher IQ is associated with a preference for more immediate rewards.If social rates of time preference are correlated with individual rates of time preference then population aging could create intergenerational asymmetries in the social rate of time preference.This phenomenon could make the optimal investments of young populations appear selfish to future generations that are older.Support from Hopkins Population Center (R24) is gratefully acknowledged. I am thankful for helpful comments received from Jim Burgess, Mike Grossman, Bruce Hamilton, Fritz Laux, Athanasios Orphanides, Katie Roche, and Eric Slade. All errors are my own. Responsible editor: Alessandro Cigno.     

The Vital Statistics Method of Estimating Net Migration By Age Cohorts

The focus of this paper is the development and testing of a method of estimating deaths which occur during a decade to aging birth and death cohorts, so that it may be possible to estimate net migration by the vital statistics (VS) method for age cohorts. Until now the VS method has been used only in making estimates of total net migration.The results obtained by using the VS method for age cohorts show that (1) the average census survival rate (CSR) method generally yields algebraically lower estimates of net migration than does the VS method; but (2) there are some striking exceptions which are apparently associated with errors in census enumeration by age, sex, and color. Comparisons between the average CSR and the VS methods are shown, by age, for both the North Carolina and the coterminous United States populations.A cursory examination of these comparisons suggests that the exclusive use of the VS method in estimating net migration for age cohorts may lead to substantial error. Finally, the magnitude of these errors in estimating net migration, as well as in census enumeration, can be roughly approximated if it is assumed that the use of the CSR method yields reasonably accurate estimates of net migration.     

A modification for use in destabilized populations of brass's technique for estimating completeness of death registration

Summary Brass has developed a method of estimating completeness of death registration using only data on deaths and population by age and sex. In this paper, his method is briefly outlined and the assumptions upon which it is based are discussed. In particular, the implications of the failure of the assumption of stability of the population are investigated. It is found that in populations where mortality has been declining, use of the technique leads to underestimation of completeness. A modification of the technique based on knowledge of the duration and rate of mortality change is proposed for use in such populations. Using simulated destabilized populations, the modification is tested and found to yield more accurate estimates of completeness of death registration than the unmodified technique. The usefulness of the modified technique is further illustrated by applying it to data for Costa Rican females in 1963.     

Estimating infant mortality trends from child survivorship data

Summary Brass's procedure for estimating mortality from census or survey data on numbers of children born to women by age group and numbers of children surviving is generalized to allow the estimation of mortality trends. The new procedure is applied to data for Costa Rica and peninsular Malaysia. The resulting infant mortality rate estimates are compared with rates calculated from vital registration figures. The comparisons suggest, surprisingly, that the estimates derived from statistics for women aged 30-50 are not noticeably inferior to those derived from those for women aged 20-30. This suggests that the common practice of disregarding statistics for women aged over 30 or 35 years may be a mistake. Figures are presented which suggest that estimates based on women aged less than 20 are likely to be very seriously biased because of differential infant mortality by age of mother at birth.     

The Network Survival Method for Estimating Adult Mortality: Evidence From a Survey Experiment in Rwanda

Adult death rates are a critical indicator of population health and well-being. Wealthy countries have high-quality vital registration systems, but poor countries lack this infrastructure and must rely on estimates that are often problematic. In this article, we introduce the network survival method, a new approach for estimating adult death rates. We derive the precise conditions under which it produces consistent and unbiased estimates. Further, we develop an analytical framework for sensitivity analysis. To assess the performance of the network survival method in a realistic setting, we conducted a nationally representative survey experiment in Rwanda (n = 4,669). Network survival estimates were similar to estimates from other methods, even though the network survival estimates were made with substantially smaller samples and are based entirely on data from Rwanda, with no need for model life tables or pooling of data from other countries. Our analytic results demonstrate that the network survival method has attractive properties, and our empirical results show that this method can be used in countries where reliable estimates of adult death rates are sorely needed.