Confounding with time

Dry Deposition. Dry deposition occurs in two steps the transport of pollutants to the earth s surface, and the physical and chemical interaction between the surface and the pollutant. The first is a fluid mechanical process (see Fluid mechanics), the second is primarily a chemical process, and neither is completely characterized at the present time. The problem is confounded by the interaction between the pollutants and biogenic surfaces where pollutant uptake is enhanced or retarded by plant activity that varies with time (47,48). It is very difficult to measure the depositional flux of pollutants from the atmosphere, though significant advances were made during the 1980s and early 1990s (49,50). 

A relatively simple example of a confounded reactor is a nonisothermal batch reactor where the assumption of perfect mixing is reasonable but the temperature varies with time or axial position. The experimental data are fit to a model using Equation (7.8), but the model now requires a heat balance to be solved simultaneously with the component balances. For a batch reactor. 

Also technical aspects may contribute to the variable incidence of parenchymal enhancement in acute human stroke. With double dose contrast medium and a time lag between injection and imaging, every fourth stroke patient showed tissue enhancement within 24 h of stroke onset (Karonen et al. 2001). Widespread parenchymal enhancement in Tl-w images seems to gradually develop over the first week when simultaneously the diffusion changes tend to pseudonormalize. Without precise clinical history confounding with tumor enhancement may occur and should be ruled out by observation of the time profile of MR contrast enhancement and DWI. 

Additional doses of benzodiazepines in long-term users are commonly needed, but it is unknown whether these additional doses have any effect. The effects of an additional 20 mg dose of oxazepam has been assessed in a double-bhnd, balanced-order, crossover, randomized study in 16 long-term users of oxazepam and 18 benzodiazepine-naive controls (5). The effects of oxazepam 10 and 30 mg were assessed on (a) saccadic eye movements as a proxy for the sedative effect (b) the acoustic startle response as a proxy for the anxiolytic effects (c) memory (d) reaction time tasks (e) subjective measurements. There were dose-related effects on the peak velocity of saccadic eye movement and response probability and on the peak amplitude of the acoustic startle response. Comparison with the controls suggested that the sedative effects might be confounded with the suppression of sedative withdrawal symptoms, whereas the patients were as sensitive as the controls to the effects of an additional dose of oxazepam on the acoustic startle response. 

Human PK is determined for the first time in Phase I, initially as a series of single doses escalated through several dose levels to establish the dose-exposure relationship and also the half-life. Because an oral dose may have an extended period of absorption, it is not possible to get the true clearance or volume of distribution from an oral dose a confounding with bioavailability may occur. Furthermore, sometimes absorption is slower... 

There is not time enough to run the whole set of experiments on one occasion. The experiments must be run during two different periods of time, maybe separated by several weeks. It is not granted that identical conditions can be obtained on these two occasions. In spite of this it is not likely that the influence of the experimental variables on the chemical reaction will be different at different times. It is other factors which are not controlled in the experiment that intervene to produce a systematic error. It would therefore be unfortunate if the systematic error should enter into the computation and bias the estimated effects of the experimental variables. This can be avoided by running the experiments divided into two blocks, one to be run on each occasion. The systematic enor can then be isolated and removed to avoid confounding with the estimated effects. 

In situ poisoning experiments were carried out on MgY zeolite by doping the toluene/methanol mbdure with either an acid (acetic acid) or a lase (3,5-dimethyl pyridine). Fresh catalysts were initially tested for about 3 h using a pure toluene/methanol mixture before introducing the doped feed. The activity was defined as a = ro/ro, where r and r(t) are the reaction rates at zero time and time t, respectively. The MgY activity diminished with time on stream when using undoped reactants because of the formation of carbonaceous deposits. Hence, when a teic confound is added into the feed, a simultaneous deactivation process by coke and poison takes place. The activity decay caused by 3,5-dimethyl pyridine (3,5-DMP) alone can not be obtained directly fiom the experimental data. To estimate the poison intrinsic effect, it can be assumed that both effects are additive, which implies that the overall deactivation rate is a simple sum of each individual rates (hypothesis of independence) [19]. According to mechanistic deactivation models [20], the overall deactivation rate is expressed as follows ... 

Of the five main effects, the one for factor 3 (equilibrium time) is clearly secondary. As for the other four, we cannot discard the possibility that some of their values are mainly due to interactions. For example, the effect of stirring (4) is confounded with interaction 12 (sample volume and temperature) while the effect of pressurization (5) is confounded with the interaction of temperature and time (23). Actually,... 

Regardless of crop type, the work rate appears to be related to exposure. This means that the number of boxes picked, crates loaded, tassels removed, etc.. Is confounded with residue levels in affecting exposure. The worker s production delimits the contact with the plant, a subject which has been studied using movies and time analysis (11), and estimated with surveys (12). Therefore, work rate data should be gathered for each subject it may explain variation In urinary or dermal exposure unaccounted for by field residues. 

Kukovecz et al. (2005) [32] reported the use of a 2 [7—4] design to study the effect of seven factors on the carbon percentage and the quality descriptor number (QDN). The factors are reaction temperature, reaction time, preheating time, catalyst mass, C2H2 volumetric flow rate, Ar volumetric flow rate and Fe MgO molar ratio. The design is a resolution III design, which means the main effects are confounded with the second-order effects. They present their results in graphs that are difficult to interpret for the main effect and interaction effect. 

We had to consider the selectivity or specificity of the stilbene synthase from rhubarb rhizomes In more detail as In this tissue the stilbene directly deriving from p-coumaric acid was hitherto unknown. Instead of this confound with a p-hydroxy group, another stilbene with an Isovanlllln structure (3-hydroxy-4-methoxy structure) is present there and already known for a long time rhapontlgenln (Fig. 4). 

Stress-relaxation measurements were attempted in the same study, but it was found that the results were confounded by fluid flow effects. When the bar is deflected by a certain amount and held in that position, the gel is compressed on the upper surface and stretched on the lower, so fluid tends to flow through the bar. At the same time, the solid phase is trying to flow to relieve the stress. The stress required to hold the bar in the deflected position decreases with time as the gel relaxes and the liquid escapes. A characteristic bend was found [73] in the relaxation curves that appeared sooner after application of the load in thinner rods, but at the same time in rods of a given size, regardless of aging time. That bend was attributed to escape of fluid from the pores (which takes longer with a thicker rod). At much shorter times, the relaxation of the load was assumed to result... 

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