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PubMed2006

The effect of sampling on estimates of lexical specificity and error rates.

Studies based on naturalistic data are a core tool in the field of language acquisition research and have provided thorough descriptions of children's speech. However, these descriptions are inevitably confounded by differences in the relative frequency with which children use words and language structures. The purpose of the present work was to investigate the impact of sampling constraints on estimates of the productivity of children's utterances, and on the validity of error rates. Comparisons were made between five different sized samples of wh-question data produced by one child aged 2;8. First, we assessed whether sampling constraints undermined the claim (e.g. Tomasello, 2000) that the restricted nature of early child speech reflects a lack of adultlike grammatical knowledge. We demonstrated that small samples were equally likely to under- as overestimate lexical specificity in children's speech, and that the reliability of estimates varies according to sample size. We argued that reliable analyses require a comparison with a control sample, such as that from an adult speaker. Second, we investigated the validity of estimates of error rates based on small samples. The results showed that overall error rates underestimate the incidence of error in some rarely produced parts of the system and that analyses on small samples were likely to substantially over- or underestimate error rates in infrequently produced constructions. We concluded that caution must be used when basing arguments about the scope and nature of errors in children's early multi-word productions on analyses of samples of spontaneous speech.

Birth Order↗
PubMed2003

An overview of the variables selection methods for the minimum sum of absolute errors regression.

The minimum sum of absolute errors regression is a robust alternative to the least squares regression whenever the errors follow a distribution for which the sample median is a more efficient estimator of location parameter than the sample mean, the errors follow a long tailed distribution, there are outliers in the values of the response variable in the data or the absolute error loss function is more appropriate than the quadratic loss function. Often an initial model may contain a large number of variables. However, in many situations, it is neither necessary nor important to include all the variables in the model. The methods for variable selection for the minimum sum of absolute errors regression are not as well documented and known as for the least squares regression. Our objective is to present an overview of the procedures to fit models with fewer variables and some criteria for selecting a model.

Algorithms↗
PubMed1993

Initial cross-sectional results from the Orinda Longitudinal Study of Myopia.

BACKGROUND: Although investigations of human refractive error development and normal ocular growth have been conducted for the last 50 years, no previous study of refractive error and the ocular components has measured all the ocular components. METHODS: The Orinda Longitudinal Study of Myopia was initiated to characterize the development of refractive error and normal eye growth in a sample of predominantly Caucasian children ages 6 to 14 years. RESULTS: Cross-sectional results from 530 children ages 5 to 12 years in the 1st, 3rd, and 6th grades are presented. CONCLUSIONS: This sample's refractive error decreased toward emmetropia with age from an average of +0.73 D at age 6 years to an average of +0.50 D by age 12 years. Between the ages of 6 and 12 years, the vitreous chamber elongated (by 0.52 mm) and the crystalline lens power decreased (by 1.35 D); surprisingly, the crystalline lens thinned by 0.14 mm during this same time period.

California↗
PubMed2002

An evaluation of measurement uncertainties in the on-line measurement of coal ash content by gamma-ray transmission.

In this paper, a significant effect producing systematic errors in the on-line measurement using gamma-ray transmission is revealed. Ash content fluctuations or thickness changes lead to a permanent negative systematic error in the results of the measurements. To study uncertainties in the measurements applicable to time-independent ash content indicators and to investigate the characteristics of the radiation attenuation process, the behavior of the quantity in question in modeled with a stationary Gaussian distribution. A systematic error-producing effect has been found, and a quantitative correction is given to compensate for it. For some other quantities in question that vary in time, a linear model is used to discuss the systematic errors in the case of automated coal gangue separator. Results of experiments that demonstrate different systematic errors for different sampling intervals are presented. The reason for these errors is the nonlinearity of the relationship between the radiation intensity, on the one hand, and the sample thickness and mass attention, on the other.

Journal Article↗
PubMed2001

Bayesian inference for categorical traits with an application to variance component estimation.

We implemented statistical models of Bayesian inference that included direct and maternal genetic effects for genetic parameter estimation of categorical traits by Gibbs sampling. The estimation errors and variances of estimates of animal versus sire and maternal grandsire models, of linear versus threshold models, of single-trait versus multiple-trait models, and of treating herd-year-season as fixed versus random effects in the model were compared. The results indicated that linear models yielded biased estimates of genetic parameters for categorical traits. The animal model was improper for analysis of categorical traits using a threshold model and the Gibbs sampler. Moreover, linear versus threshold models and animal versus sire-maternal grandsire models resulted in larger Monte Carlo errors and increased auto-correlations among posterior samples. Treating herd-year-seasons as random effects in the threshold models decreased the Monte Carlo error, auto-correlations, and the variances of estimates. Efficiency of the single-trait threshold sire model, as measured by the variance of the estimates, was lower than for a multiple-trait model that included a correlated continuous trait, but both estimates were unbiased. Therefore, the threshold single-trait sire and maternal grandsire model is a feasible alternative to the multiple-trait model for analysis of variance components of categorical traits affected by direct and maternal genetic factors.

Animals↗
PubMed2000

Qualitative block design analysis in posttraumatic stress disorder.

Qualitative and quantitative Block Design performance was examined in Vietnam combat veterans with PTSD diagnoses (n = 23) and Vietnam combat veterans without PTSD or other mental disorders diagnoses (n = 19). Results indicated that PTSD-diagnosed veterans committed more single block rotations than the comparison sample, and that their errors occurred more frequently in right hemispace than errors made by the comparison sample. The two groups did not differ in the number of configural errors made, errors committed in left hemispace, or in quantitative performance measures. Findings are suggestive of relative left hemisphere hypoactivation and are congruent with prior research documenting cerebral asymmetries in emotional disorders.

Affect↗
PubMedDate not supplied

Sample sizes needed for specified margins of relative error in the estimates of the repeatability and reproducibility standard deviations.

Sample size formulas are developed to estimate the repeatability and reproducibility standard deviations (Sr and S(R)) such that the actual error in (Sr and S(R)) relative to their respective true values, sigmar and sigmaR, are at predefined levels. The statistical consequences associated with AOAC INTERNATIONAL required sample size to validate an analytical method are discussed. In addition, formulas to estimate the uncertainties of (Sr and S(R)) were derived and are provided as supporting documentation. Formula for the Number of Replicates Required for a Specified Margin of Relative Error in the Estimate of the Repeatability Standard Deviation.

Chemistry Techniques, Analytical↗
PubMed1999

Apolipoprotein B signal peptide polymorphism distribution among south Amerindian populations.

We report the distribution of the APOB signal peptide polymorphism in 5 native populations of South America: 2 samples of Mataco and 1 sample each of Pilagá and Toba from the Argentinian Chaco and 1 sample of Ache from the Paraguay forest. A randomly selected subsample of a previously studied sample from the Cayapa of Ecuador (Scacchi et al. 1997) was reanalyzed to investigate probable differences attributable to sampling, laboratory techniques, or interobserver error. The polymorphism observed in the signal peptide region of the APOB gene among native populations of South America exhibits the same range of variation found among geographic continental populations, confirming the high genetic heterogeneity of South Amerindians. Extremes in the allele prevalences were found among the Mataco and Ache, populations not far apart geographically. The small differences in genotype and allele frequencies between the subsample of the Cayapa analyzed here and the original Cayapa sample and between the 2 Mataco samples were not statistically significant and most likely were due to sampling error.

Alleles↗
PubMedDate not supplied

Modeling the postdialysis rebound: the reconciliation of current formulas.

Three approaches are currently used in kinetic models (UKMs) to account for the postdialysis rebound in urea concentration, and thereby accurately measure the hemodialysis dose, KT/V (where K, T, V denote dialyzer clearance, dialysis duration, and urea distribution volume, respectively). The approach developed by Smye uses an intradialytic sample to predict the postdialysis equilibrium concentration, Ce, which is then used in a single pool UKM to give KT/V. A second approach developed by Tattersall introduces a patient clearance time, tp. The true dialysis dose is then given by T/(T + tp) x apparent dose, and tp is estimated to be 36 minutes. The Daugirdas analysis uses an empiric regression equation to give the true dose; KT/V)true from the single pool value, KT/V)sp; KT/V)true = KT/V)sp - (36/T)(KT/V)sp + 0.03. The analysis confirms the equivalence of all three formulas, which arises from the observation that during the later stages of dialysis, the urea concentration decreases as a single exponential. The formulas are independent of whether a flow or diffusion model is used to describe the kinetics of urea removal. The original analysis assumed constant volumes, but the effect of ultrafiltration volume u on C(e) may be accounted for by multiplying by (1 + u/V). The Smye equation is more vulnerable to error in practice, because small errors in the intradialytic sample give larger errors in the equilibrium concentration estimate, whereas dose estimates based on the Tattersall and Daugirdas equations are less affected by sampling errors. However, unlike the Smye approach, these two formulas would need adaptation for use with other solutes. The advent of continuous urea monitoring should permit more accurate, prospective estimates of equilibrium concentrations and dialysis dose.

Blood Flow Velocity↗
PubMed2006

Diurnal variability in concentrations and sources of Escherichia coli in three streams.

Microbial contamination is a major concern for drinking water worldwide. Many monitoring protocols that use one or very few samples are inadequate and introduce a very large margin of error. An intensive sampling program needs to be conducted to characterize the Escherichia coli concentrations of a source water stream prior to establishing a monitoring program so that the sample frequency can be determined statistically, based on an acceptable margin of error. Developing meaningful monitoring programs for managing bacterial water quality is dependant on scientific data that determine the bacterial sources. In this study, three streams from drinking water watersheds were sampled every 15 min over a 24 h period on three different days to determine the concentrations of E. coli and to identify their sources, using ribosomal RNA finger printing (ribotyping). The concentrations of E. coli varied throughout the day in each of the three streams. Ribotyping identified many different animal sources of E. coli in the samples. The sources of E. coli varied significantly with stream (P < 0.001, df = 16). The development of monitoring programs for watersheds needs to consider the watershed, and care needs to be taken in selecting appropriate sample sites, sampling regime, and number of samples taken during each sampling period. This note provides a prescription for the development of monitoring programs for watersheds.

Animals↗
PubMed2004

Maximum a posteriori Bayesian estimation of epirubicin clearance by limited sampling.

AIMS: To develop a limited sampling strategy for estimation of epirubicin clearance. METHODS: The data set comprised 1051 concentrations measured in 105 patients with advanced or metastatic breast cancer treated with epirubicin alone. Ten limited sampling designs comprising two or three blood samples were proposed, taken at times identified by D-optimality from population pharmacokinetic parameter estimates. The data set was then truncated to include the sampling times for each of the designs. MAP Bayesian estimates of clearance were generated for each design and compared with clearance estimates obtained using all the data. The limited sampling designs were also validated using a separate data set obtained from 18 patients with either breast cancer or hepatocellular carcinoma. The sensitivity of the best limited sampling designs to sample time recording errors of 0-10% or 10-20% was then assessed using a simulated data set including 200 patients. RESULTS: The optimum sampling times were: end of the injection and 18 min, 40 min, 3 h, 10 h and 48 h after the start of the injection. The best three-sample design included samples at 40 min, 3 h and 48 h and gave unbiased estimates of clearance with an imprecision of 9.1% (95% CI 7.3, 10.5). The best two sample design included samples at 3 and 48 h and gave unbiased estimates of clearance with an imprecision of 12.4% (95% CI 9.6, 14.6). Using the validation data set, these two and three sample designs gave unbiased estimates of clearance with an imprecision of 5.6% (95% CI 3.7, 7.0) and 4.2% (95% CI 2.6, 5.3), respectively. Simulations that included 0-10% or 10-20% errors in the recording of the blood sampling times had negligible effects on the bias and imprecision of clearance estimates. CONCLUSIONS: Limited sampling designs have been identified and validated that estimate epirubicin clearance with adequate precision and without bias from two or three blood samples. These designs also allow flexibility in blood sample collection and are robust with regard to sample time recording errors.

Adult↗
PubMed2007

Checking the consistency of the two blood samples slope-intercept method for estimating GFR using the single blood sample formula in children.

BACKGROUND: The slope-intercept method is widely used for the determination of the plasma clearance of 51Cr-EDTA. When three or more plasma samples are used, the goodness-of-fit (r2) can be used as a measure of consistency of the samples. This parameter can not be used, however, if only two samples are available. PURPOSE: To evaluate whether the single-sample technique (SBS) can be used to check the consistency of the slope-intercept method using two blood samples (2BS) in children. METHODS: Simulated computer models of a mono-exponential curve were created in order to represent three children aged 3, 6 and 10 years, each with a large range of clearances values and three distribution volumes, respectively 20%, 25% and 30% of body weight. Errors were then introduced in the injected dose (errors from -50% to +50%) and on the 120 or 240 min blood sample (errors from -50% to +50%). The effects of these errors on the clearance measurement using 2BS and SBS methods were calculated and compared. RESULTS: The errors on the injected dose, the 120 min and 240 min plasma samples introduced errors in the same direction and with the same magnitude on both the SBS and 2BS clearance values. For that reason, the comparison between the SBS methods and the 2BS techniques has a low sensitivity in detecting an eventual error. Striking differences between the SBS method and the 2BS technique were only observed when considerable errors on the injected dose or plasma samples were introduced, particularly in case of a reduced clearance. The comparison between the SBS clearances calculated using the 120 min sample to that obtained using the 240 min samples is slightly more sensitive. However, this approach is also slightly less specific. A difference of more than 10 ml . min(-1). 1.73 m(-2) can be observed in the absence of an error. CONCLUSIONS: The use of the SBS for checking the consistency of the 2BS constitutes an insensitive approach to detect an eventual error in the injected dose or in the plasma samples. Obvious different results obtained by SBS and 2BS or between the SBS calculated using the 120 min and the 240 min samples suggest the presence of an error, but comparable results do not exclude erroneous measurement. Moreover, a difference of more than 10 ml . min(-1). 1.73 m(-2) can be observed in the absence of an error in the injected dose or in the plasma samples.

Body Size↗
PubMed2005

Optimization and validation of a fully automated silica-coated magnetic beads purification technology in forensics.

Automated procedures for forensic DNA analyses are essential not only for large-throughput sample preparation, but are also needed to avoid errors during routine sample preparation. The most critical stage in PCR-based forensic analysis is DNA isolation, which should yield as much highly purified DNA as possible. The extraction method used consists of pre-treatment of stains and samples, cell lysis using chaotropic reagents, binding of the DNA to silica-coated magnetic particles, followed by elution of the DNA. Our work focuses mainly on sample preparation, obtaining the maximum possible amount of biological material from forensic samples, and the following cell lysis, to create a simple standardized lysis protocol suitable for nearly all forensic material. After optimization and validation, the M-48 BioRobot((R)) workstation has been used for more than 20,000 routine lab samples. There has been no evidence of cross contamination. Resulting DNA from as small as three nuclear cells yield reliable complete STR amplification profiles. The DNA remains stable after 2 years of storage.

Bone and Bones↗
PubMed2003

[Prevalence of psychiatric pathology at a rural health centre].

OBJECTIVES: To find the prevalence of psychiatric pathology at a rural health centre and to compare the results with results from studies with a similar design. DESIGN: Two-stage crossover study of prevalence: first, detection and then, psychiatric pathology evaluation.Setting. El Marquesado Health Centre (Granada). Scattered rural population with low social and economic level and high percentage of elderly people. PARTICIPANTS: Simple randomised sample representing the population over 18. N=245; error alpha=5%. Sampling source: electoral register. MEASUREMENTS: GHQ-28 Goldberg Questionnaire, social and demographic data, and CAGE test. The CIS questionnaire for psychiatry was administered to those who exceeded the cut-off point (GHQ-28=5) and they were allocated a DSM IV diagnosis. If CAGE was>=2, damaging alcohol consumption was studied. If there was a prior diagnosis, the second stage was avoided. Measurements of prevalence with their exact confidence intervals were made. RESULTS: Prevalence of psychiatric pathology: 28.6% (34.5% in women and 22.6% in men). 30% of cases had two diagnoses. Anxiety disorders were the most common pathology (8.97%), representing 23.9% of the grouped diagnoses. Depression disorders had 8.16% prevalence; problematic consumption of substances, 6.12%. The most common isolated diagnosis was Dysthymia (9.78% of cases). Hidden psychiatric morbidity reached 40%. CONCLUSIONS: The prevalence of psychiatric pathology is very high. The most common disorders are those of anxiety, followed by depression and damaging alcohol consumption.

Adolescent↗
PubMed1991

Time sampling: a methodological critique.

Numerous developmental studies of social behavior employ an observational technique called time sampling. This technique is used to estimate the frequencies and proportions of time that actual behaviors occur and to measure group or individual differences. By contrasting time sampling to continuous sampling, we demonstrate that (a) time sampling is inaccurate for estimating actual durations or frequencies of behavior, and (b) resulting individual and/or group differences can change depending on whether time sampling or continuous sampling is used. Error rates are particularly high when the total bout lengths of the actual behaviors are short (as in mother-infant interactions) and/or when interval length is long. Because the majority of mother and infant behaviors are short in bout length (less than 60 sec), when percent error is the greatest, we suggest that alternative sampling methods that have proven validity be used in future studies (e.g., scan, point, or continuous sampling).

Behavior↗
PubMedDate not supplied

Statistical analysis in family medicine research.

Analysis of the data is an important stage in any research project. Statistical analysis allows us to summarize data with easily understood concepts, such as rates, means, medians, or standard deviations. Statistical analysis also allows us to judge whether differences or associations identified in the sample are merely the result of the random error associated with sampling, and it often provides a measure of the strength of the associations identified. While appropriate statistical analysis is crucial to any research endeavor, the success or failure of the project is more dependent upon what has been done prior to beginning data analysis, not after. This article reviews concepts fundamental to any statistical analysis, the primary reasons to submit data to analysis, and selection of the appropriate statistical test. The discussion is brief of necessity, and readers are encouraged to consult standard statistical texts for more detailed information.

Adult↗
PubMed2003

Population size estimation in Yellowstone wolves with error-prone noninvasive microsatellite genotypes.

Determining population sizes can be difficult, but is essential for conservation. By counting distinct microsatellite genotypes, DNA from noninvasive samples (hair, faeces) allows estimation of population size. Problems arise because genotypes from noninvasive samples are error-prone, but genotyping errors can be reduced by multiple polymerase chain reaction (PCR). For faecal genotypes from wolves in Yellowstone National Park, error rates varied substantially among samples, often above the 'worst-case threshold' suggested by simulation. Consequently, a substantial proportion of multilocus genotypes held one or more errors, despite multiple PCR. These genotyping errors created several genotypes per individual and caused overestimation (up to 5.5-fold) of population size. We propose a 'matching approach' to eliminate this overestimation bias.

Alleles↗
PubMed1985

Errors in measuring water potentials of small samples resulting from water adsorption by thermocouple psychrometer chambers.

The adsorption of water by thermocouple psychrometer assemblies is known to cause errors in the determination of water potential. Experiments were conducted to evaluate the effect of sample size and psychrometer chamber volume on measured water potentials of leaf discs, leaf segments, and sodium chloride solutions. Reasonable agreement was found between soybean (Glycine max L. Merr.) leaf water potentials measured on 5-millimeter radius leaf discs and large leaf segments. Results indicated that while errors due to adsorption may be significant when using small volumes of tissue, if sufficient tissue is used the errors are negligible. Because of the relationship between water potential and volume in plant tissue, the errors due to adsorption were larger with turgid tissue. Large psychrometers which were sealed into the sample chamber with latex tubing appeared to adsorb more water than those sealed with flexible plastic tubing. Estimates are provided of the amounts of water adsorbed by two different psychrometer assemblies and the amount of tissue sufficient for accurate measurements of leaf water potential with these assemblies. It is also demonstrated that water adsorption problems may have generated low water potential values which in prior studies have been attributed to large cut surface area to volume ratios.

Journal Article↗