Rather than visually degrading the stimuli, target selection difficulty was manipulated by rotation of the targets and/or the distractors, thus keeping the visual quality of the stimuli intact, but rendering selection of targets a more time-consuming process [11], [26], [27].
The road to Poland and Ukraine has been bumpier in terms of selection difficulties.
Variability in the permeability of portions of the genome subject to different levels of selection highlights the difficulty of delineating species when hybridization is common (Harrison, 1990).
Limitations of this study include potential selection bias, and difficulty of participants describing risks of a novel and unfamiliar technology.
Instead even options unavailable for choice (which could not affect the difficulty of choice selection) affected activity.
Activity in the ACC could not be explained by the hypothesis that only options available for choice (and which might therefore affect the difficulty of choice selection) determined activity levels.
In summary, we acknowledge the common difficulty of selection bias in this type of research and we attempted to control for obvious individual patient selection bias, while recognizing the limits of generalization imposed by the sample.
Our structure selection results highlight the difficulty of reliable structure prediction by first-principles calculations.
However, due to the scarcity of drought and salt tolerance alleles in the peanut gene pool, the quantitative nature of abiotic stress tolerance, and the difficulty of selection, peanut stress resistance breeding can be a time consuming, labor-intensive, and difficult process [ 6].
Therefore, detecting unambiguous examples of positive selection has been difficult due the difficulty of many methods to differentiate between positive selection and demographic history.
