Observational selection bias

Observational selection bias - Believing that events are increasing in frequency when in effect we are simply noticing them more. 

These results implied that the MPA polymer responded in two different ways to the valence of the cations tested negative Cotton effects for monovalent ions and positive effects for divalent ions, thus behaving as a valence sensor for metal cations. No relationship between ionic radii and the observed selectivity bias was found and, as expected, the enantiomeric polymer [poly-(S )-2] gave the opposite results. 

In this review we point out briefly the various instrumentation factors important in obtaining mass spectra of inorganic and organometallic compounds and in affecting the quality of spectra observed. Selected examples are chosen, admittedly with a personal bias, to illustrate the various types of information available from mass spectra. We have... 

As discussed by Hood and Miller (7), and in the lecture by Hood at the Maternal Toxicology Symposiums in 2009 (summarized in Beyer et al. (8)), it can be argued that it is Khera s interpretation, rather than the developmental toxicity study results themselves, that may be of concern. Khera s literature review indicated a possible association between maternal toxicity and embryo-fetal effects, but it did not establish a causal relationship between these two observations. Additional criticisms of Khera s hypothesis include the fact that his literature review was retrospective, there was a potential selection bias arising from the general tendency not to publish negative data, and the failure to adequately address maternal toxicity endpoints in the published literature of the time. In fact, Khera himself stated that in 40% of the studies he evaluated in support of his hypothesis the maternal toxicity data were insufficient or nonexistent (9). 

A recent study of group selectivity in iodocyclizations which could form either ds-fused tetrahydrofu-ran or 7-lactone products (equation IS) has shown that the observed selectivity correlates with the conformational bias of each isomeric substrate.62 One isomer, with no significant conformational bias, produced a mixture of the products upon cyclization of the ester, but gave only the 7-lactone upon cyclization of the carboxylic acid. 

Reduces selection bias and observer bias (nonrandom error). 

Regional cerebral blood flow was examined using Doppler ultrasound in 15 workers exposed to 3.2-28.9 ppm carbon disulfide for a mean of 20 years (Aaserud et al. 1992). Studies were performed 4 years after exposure ended. Asymmetrical blood flow patterns were observed in 8/14 workers, all of whom had encephalopathies consistent with carbon disulfide exposure. However, when the results were corrected to adjust for a possible influence of pC02, the values did not differ between the exposed workers and referents. No clear conclusions about this study can be made because of the small number of exposed workers, lack of a current exposure group, possible selection bias, and age variation between cohort and referents. 

The authors note that the munber of SRS subjects who were healthy and willing to participate in an exposure study was small relative to the numbers cited by community groups as adversely affected by MTBE. Thus, there was likely to have been a self-selection bias that the authors suggest would have been in the direction of deterring even more sensitive individuals from participating. They also state that time constraints prevented the use of additional neurobehavioural tests or other objective markers (e.g., tear film) that may have been more sensitive. However, tear film break-up tests had been incorporated in the chamber studies of [30,56,57] in which no effect of exposure to MTBE was observed on this parameter. 

The poor stability of some semiconducting polymers and their sensitivity to external factors often make it difficult to deduce true intrinsic properties of a certain material [101,102]. Most commonly, the off-currents in a TFT are governed by extrinsic factors and it also has an impact on subthreshold slope, threshold voltages, and bias stress. In fact, this sensitivity prompted some researchers to use OSC materials as gas and chemical sensors in TFT devices [103,104]. The observed selective sensitivity is related either to the chemical nature of the semiconductor functionalities or the interaction of the grain... 

Moreover, the observation of nanoparticles in a conventional TEM and high resolution must be done very carefully and held in various magnifications, in order to avoid a particle selection bias. This bias can distort the interpretation of the morphological and structural material, which can be avoided by a prior analysis inSEM. 

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