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Frequency Measurements (frequency + measurement)

Distribution by Scientific Domains

Life Sciences	40%

Selected Abstracts

Impact and Quantification of the Sources of Error in DNA Pooling Designs

ANNALS OF HUMAN GENETICS, Issue 1 2009
A. Jawaid
Summary The analysis of genome wide variation offers the possibility of unravelling the genes involved in the pathogenesis of disease. Genome wide association studies are also particularly useful for identifying and validating targets for therapeutic intervention as well as for detecting markers for drug efficacy and side effects. The cost of such large-scale genetic association studies may be reduced substantially by the analysis of pooled DNA from multiple individuals. However, experimental errors inherent in pooling studies lead to a potential increase in the false positive rate and a loss in power compared to individual genotyping. Here we quantify various sources of experimental error using empirical data from typical pooling experiments and corresponding individual genotyping counts using two statistical methods. We provide analytical formulas for calculating these different errors in the absence of complete information, such as replicate pool formation, and for adjusting for the errors in the statistical analysis. We demonstrate that DNA pooling has the potential of estimating allele frequencies accurately, and adjusting the pooled allele frequency estimates for differential allelic amplification considerably improves accuracy. Estimates of the components of error show that differential allelic amplification is the most important contributor to the error variance in absolute allele frequency estimation, followed by allele frequency measurement and pool formation errors. Our results emphasise the importance of minimising experimental errors and obtaining correct error estimates in genetic association studies. [source]

Temporal dynamics of dissolved oxygen in a floating,leaved macrophyte bed

FRESHWATER BIOLOGY, Issue 8 2008
KARA GOODWIN
Summary 1. Oxygen concentrations in shallow vegetated areas of aquatic systems can be extremely dynamic. In these waters, characterizing the average oxygen content or frequency of low oxygen events (hypoxia) may require high frequency measurements that span seasons and even years. In this study, moored sondes were used to collect 15-min interval dissolved oxygen (DO) readings in an embayment of the tidal Hudson River with dense coverage by an invasive floating leaved plant (Trapa natans) and in adjacent open waters. Measurements were made from late spring to summer over a 2-year period (2005, 2006). 2. Oxygen concentrations were far more dynamic in the vegetated embayment than in the adjacent open waters and while hypoxic conditions never occurred in the open waters, they occurred frequently in the vegetated site. Overall the vegetated site was hypoxic (DO < 2.5 mg L,1) 30% of the time and had an average oxygen concentration of 5.1 mg L,1. Oxygen concentration was significantly (P < 0.0001, anova) related to season, year and tide. Low tide periods during summer of 2006 had the lowest average oxygen concentration and the highest frequency of hypoxia. 3. The greater hypoxia in summer than spring is related to changes in plant morphology. In the spring and early summer when plants are submersed hypoxia occurs at lower frequency and duration than in the summer when dense floating vegetation covers the water. The tidal pattern in oxygen is related to hydrologic exchange with the non-vegetated open waters. Year-to-year variation may be related to relatively small changes in plant biomass between years. 4. Oxygen concentrations in aquatic systems can be critical to habitat quality and can have cascading impacts on redox sensitive nutrient and metal cycling. For some systems with dynamic oxygen patterns neither weekly spot sampling nor short-duration, high-frequency measurements may be sufficient to characterize oxygen conditions of the system. [source]

A Wet/Wet Differential Pressure Sensor for Measuring Vertical Hydraulic Gradient

GROUND WATER, Issue 1 2010
Brad G. Fritz
Vertical hydraulic gradient is commonly measured in rivers, lakes, and streams for studies of groundwater,surface water interaction. While a number of methods with subtle differences have been applied, these methods can generally be separated into two categories; measuring surface water elevation and pressure in the subsurface separately or making direct measurements of the head difference with a manometer. Making separate head measurements allows for the use of electronic pressure sensors, providing large datasets that are particularly useful when the vertical hydraulic gradient fluctuates over time. On the other hand, using a manometer-based method provides an easier and more rapid measurement with a simpler computation to calculate the vertical hydraulic gradient. In this study, we evaluated a wet/wet differential pressure sensor for use in measuring vertical hydraulic gradient. This approach combines the advantage of high-temporal frequency measurements obtained with instrumented piezometers with the simplicity and reduced potential for human-induced error obtained with a manometer board method. Our results showed that the wet/wet differential pressure sensor provided results comparable to more traditional methods, making it an acceptable method for future use. [source]

Resonance frequency measurements in vivo and related surface properties of magnesium-incorporated, micropatterned and magnesium-incorporated TiUnite®, Osseotite®, SLA® and TiOblast® implants

CLINICAL ORAL IMPLANTS RESEARCH, Issue 10 2009
Young-Taeg Sul
Abstract Objective: To investigate implant stability using resonance frequency measurements of topographically changed and/or surface chemistry-modified implants in rabbit bone. Material and methods: Six groups of microstructured, screw-shaped titanium implants: two oxidized, cation-incorporated experimental implants [Mg implants and MgMp implants with micropatterned thread flanges (80,150 ,m wide and 60,70 ,m deep)] and four commercially available clinical implants (TiUnite®, Osseotite®, SLA®, and TiOblast®) were installed in 10 rabbit tibia for 6 weeks. The surface properties of the implants were characterized in detail using several analytical techniques. Implant stability was measured using a resonance frequency analyzer (OsstellÔ). Results: Surface characterization of the implants revealed microstructured, moderately rough implant surfaces varying 0.7,1.4 ,m in Sa (mean height deviation), but with clear differences in surface chemistry. After 6 weeks, all implants showed statistically significantly higher increases in implant stability. When compared with one another, MgMp implants showed the most significant mean implant stability quotient (ISQ) value relative to the others (P,0.016). In terms of increment (,ISQ) in implant stability, MgMp implants showed a significantly greater value as compared with Osseotite® (P,0.005), TiOblast® (P,0.005), TiUnite® (P,0.005), SLA® (P,0.007), and Mg implants (P,0.012). In addition, transducer direction dependence of resonance frequency analysis (RFA) measurements was observed such that the differences in the mean ISQ values between longitudinal and perpendicular measurements were significant at implant placement (P,0.004) and after 6 weeks (P,0). Conclusion: The present study found that implant surface properties influence RFA measurements of implant stability. Surface chemistry-modified titanium implants showed higher mean ISQ values than did topographically changed implants. In particular, cation (magnesium)-incorporated micropatterns in MgMp implants may play a primary role in ,ISQ. [source]