Correction to: Trend estimation for complex survey designs of water chemistry indicators from Sierra Nevada Lakes

The original version of this article contained a misaligned equation. The following equation replaces the online printed on the 5th page of the article.

Trend estimation for complex survey designs of water chemistry indicators from Sierra Nevada Lakes

Surveys for long-term monitoring programs managing natural resources often incorporate sampling design complexity. However, design weights are often ignored in trend models of data from complex sampling designs. Generalized random tessellation stratified samples of a simulated population of lakes are selected with various levels of survey design complexity, and three trend approaches are compared. We compare an unweighted trend model, linear regression models of the trend in design-based estimates of annual status, and a probability-weighted iterative generalized least squares (PWIGLS) approach with a linearization variance. The bias and confidence interval coverage of the trend estimate and the size and power of the trend test are used to evaluate weighted and unweighted approaches. We find that the unweighted approach often outperforms the other trend approaches by providing high power for trend detection and nominal confidence interval coverage of the true trend regression parameter. We also find that variance composition and revisit design structure affect the performance of the PWIGLS estimator. When a subpopulation exhibiting an extreme trend is sampled disproportionately to its occurrence in the population, the unweighted approach may produce biased estimates of trend with poor confidence interval coverage. We recommend considering variance composition and potential subpopulation trends when selecting sampling designs and trend analysis approaches.

Standard and digestive metabolism in the banded water snake, Nerodia fasciata fasciata

Estimating energy costs by respirometry is fundamental to many studies of the ecology, behavior and evolution of reptiles. However, traditional respirometry procedures seldom incorporate objective techniques for removal of outliers from estimates of metabolic parameters. We demonstrate how computer-automated respirometry equipment, which records many respiratory measurements over short intervals, can be coupled with mathematical procedures to produce robust estimates of pre- and post-prandial metabolism in banded water snakes (Nerodia fasciata fasciata). Standard metabolic rate of N. f. fasciata was estimated to be 1.21 ml O2/h (mass = 30.21 +/- 0.74 g) at 25 degrees C. After ingestion of a fish equaling 20% of their body mass, snakes exhibited a fivefold increase in metabolic rate with peak O2 consumption rate (VO2) reaching 6.5 ml O2/h. Total cost of digestion was 5.44 kJ, equivalent to approximately 21% of the energy in the meal. Repeated measurements of metabolism in the same individuals revealed that our methods yielded similar results, even when individuals exhibited different patterns of VO2 variation between respiratory trials. Our results underscore the importance of obtaining many VO2 measurements, coupled with objective removal of outlier values from estimates of metabolic rate, especially when metabolic values are to be interpreted in a comparative context.

Lipoproteins as predictors of ischemic heart disease in non-insulin-dependent diabetic men

In order to determine whether low levels of high-density lipoprotein cholesterol (HDL-C), which are predictive of ischemic heart disease in the general population, can also predict death from ischemic heart disease among diabetic men, we contrasted lipoprotein and other heart disease risk factors in 62 men with non-insulin-dependent diabetes mellitus, 14 of whom died of ischemic heart disease during a 12-year follow-up period. Compared to all other diabetic men, those who died of ischemic heart disease were older, had higher levels of fasting plasma glucose (FPG) total plasma cholesterol, and triglycerides, lower HDL-C levels, and higher low-density lipoprotein cholesterol (LDL-C) levels and were more likely to have been cigarette smokers; only total cholesterol, LDL-C, and the LDL/HDL ratio were statistically significant. Age, FPG, total plasma cholesterol, and LDL-C were all independently predictive of fatal heart disease by multivariate analysis. Neither HDL-C nor the LDL/HDL ratio predicted ischemic heart disease death better than the total plasma cholesterol or LDL-C. The use of HDL-C, LDL-C, or total plasma cholesterol level in the model did not eliminate the significant association with FPG, which suggests that the noxious effect of hyperglycemia is independent of the changes in blood lipids.

Population-based study of glycosylated hemoglobin, lipids, and lipoproteins in nondiabetic adults

Glycosylated hemoglobin levels reflect glucose homeostasis over the preceding months. Many investigators have reported levels of glycosylated hemoglobin in diabetics and have noted a strong correlation of these levels with lipids and lipoproteins, but not with HDL cholesterol. We report here the first population-based study of the predominant fraction of glycosylated hemoglobin, HbA1, and its correlates in nondiabetics. In 558 euglycemic adults (fasting plasma glucose less than 140 mg/dl) aged 40 to 79 years who had no history of diabetes, HbA1 was normally distributed, was unrelated to age, and was correlated with fasting plasma glucose in men and women and with obesity in women. After adjusting for age and obesity, HbA1 was significantly correlated with total plasma cholesterol and LDL cholesterol in both sexes and with total plasma triglyceride and VLDL cholesterol in men. HDL cholesterol was not significantly associated with HbA1 in men or women. These associations are similar, although weaker, than those reported in most studies of diabetics. The finding that the association of glycosylated hemoglobin with lipids and lipoproteins extends throughout the normal range of blood sugar suggests that this association may be relevant to both the excess risk of ischemic heart disease in diabetics and in nondiabetics with higher levels of fasting plasma glucose.

Fasting plasma glucose, uric acid, and triglycerides as predictors of the ratio of total cholesterol to HDL-C

It was reported previously that a score composed of glucose, uric acid, and triglyceride was correlated closely with the total cholesterol:high density lipoprotein cholesterol (TC:HDL) ratio and could be used to predict those subjects for whom the more expensive HDL measurement would be useful. The authors tested this score using multiple regression and discriminant function analysis in a population sample of 209 men aged 40-79 years, Triglyceride was the only component of the score that contributed to the prediction or classification of the TC:HDL ratio. Although the correlations of the score with the TC:HDL ratio were relatively good (r = 0.72 or greater), the discriminant function analysis showed that subjects with high or low scores could have high or low TC:HDL ratios. The authors conclude that the score (essentially the triglyceride level) is inadequate to permit the classification of any individual as having a definite high or low TC:HDL ratio.