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Sampling Fraction (sampling + fraction)

Distribution by Scientific Domains

Medical Sciences	50%

Selected Abstracts

Nondestructive, Stereological Estimation of Canopy Surface Area

BIOMETRICS, Issue 1 2010
Dvoralai Wulfsohn
Summary We describe a stereological procedure to estimate the total leaf surface area of a plant canopy in vivo, and address the problem of how to predict the variance of the corresponding estimator. The procedure involves three nested systematic uniform random sampling stages: (i) selection of plants from a canopy using the,smooth fractionator, (ii) sampling of leaves from the selected plants using the,fractionator, and (iii) area estimation of the sampled leaves using,point counting. We apply this procedure to estimate the total area of a chrysanthemum (Chrysanthemum morifolium L.) canopy and evaluate both the time required and the precision of the estimator. Furthermore, we compare the precision of point counting for three different grid intensities with that of several standard leaf area measurement techniques. Results showed that the precision of the plant leaf area estimator based on point counting is high. Using a grid intensity of 1.76 cm2/point we estimated plant and canopy surface areas with accuracies similar to or better than those obtained using image analysis and a commercial leaf area meter. For canopy surface areas of approximately 1 m2 (10 plants), the fractionator leaf approach with sampling fraction equal to 1/9 followed by point counting using a 4.3 cm2/point grid produced a coefficient of error of less than 7%. The,smooth fractionator,can be used to ensure that the additional contribution to the estimator variance due to between-plant variability is small. [source]

A Case-Control Follow-up Study for Disease-Specific Mortality

BIOMETRICS, Issue 1 2003
Marshall M. Joffe
Summary. Case-control studies often rely on subjects to report their own screening or exposure information; this information is often obtained from cases after the event of interest has occurred. This is problematic for mortality outcomes, because dead subjects cannot report the desired information. To avoid this problem, Weiss and Lazovich (1996, American Journal of Epidemiology143, 319,322) proposed obtaining exposure or screening information from potential cases, i.e., subjects diagnosed with disease, at the time of disease diagnosis, and also from a referent series. The design is best viewed as a new scheme for sampling from a cohort. I review estimation of the effects of time-varying screening or exposure in cohort studies, using a new factorization. I then show how this factorization, together with ignorability assumptions, allows valid estimation from these new designs. Even when the sampling fraction of nondiseased subjects is unknown, causal risk ratios are estimable if diagnosis is rare in the cohort. I illustrate and compare conventional and new methods with data from the Health Insurance Plan study. [source]

Factors influencing testicular volume in young men: results of a community-based survey

BJU INTERNATIONAL, Issue 4 2002
J.H. Ku
Objective,To investigate the factors influencing testicular volume in young men in the community. Subjects and methods,Between May and November 2001, 2700 men aged 20 years and dwelling in the community were randomly selected at a 10% sampling fraction after a sampling process by census district; 2080 men agreed to participate in the study. All volunteers underwent a standard evaluation, including a detailed medical history and physical examination. After excluding those with testicular diseases the study comprised 1792 men. Results,There were significant but weak correlations between testicular volumes and height, body weight and body mass index. In a multivariate model, high environmental temperature was associated with a decreased likelihood (odds ratio, OR, 0.42; 95% confidence interval, CI, 0.29,0.60; P < 0.001) of a paired testicular volume being below the 25th percentile of all participants. The likelihood of a low paired testicular volume varied by area, with a 1.6-fold greater risk in men dwelling in large rural areas than in those in major towns. Increasing height was associated with a decreased likelihood (OR 0.60; 95% CI 0.38,0.96; P = 0.032) and low body weight with an increased likelihood of a low paired testicular volume (OR 2.54; 95% CI 1.57,4.12; P < 0.001). Conclusion,These results establish that demographic and environmental factors have an effect on testicular size and suggest that body size may be important in determining testicular size in late adolescents. [source]

Sampling for a longitudinal study of the careers of nurses qualifying from the English pre-registration Project 2000 diploma course

JOURNAL OF ADVANCED NURSING, Issue 4 2000
Louise Marsland PhD BSc RMN
Sampling for a longitudinal study of the careers of nurses qualifying from the English pre-registration Project 2000 diploma course This paper describes the processes involved in selecting a sample, from the eight English regional health authorities, of nurse qualifiers from all four branches of the Project 2000 pre-registration diploma course, for a longitudinal study of nurses' careers. A simple random sample was not feasible since accurate information about the population could not be obtained and the study design involved recruiting participants by personal visit. A multi-stage approach was therefore adopted in which ,college of nursing' was taken as the primary sampling unit. Sampling was further complicated by the fact that adult branch students could generally only be visited in larger groups than was ideal. Information obtained during pilot work about the accuracy of data about the population, course completion rates and the proportion of students who were likely to agree to participate was used to calculate required sampling fractions. The final sample was therefore a function of this information and the practicalities of recruiting nurses into the study. [source]