In the traditional three-dimensional CT reconstruction algorithms, high-frequency characteristics are difficult to obtain, and the noise is difficult to filter.
Postoperative changes and tumour recurrence, especially in the early period, may have similar morphologic characteristics, being difficult to differentiate using CT or MRI.
As matching directly on observable characteristics is difficult if the set of potentially relevant characteristics is large, matching on propensity scores has been proposed as a valid method (Rosenbaum and Rubin, 1983).
Although the four characteristics are difficult to define and measure for making service quality, there are three representative methods of measuring service quality in the past studies, as follows.
Because chronic obstructive pulmonary disease (COPD) is a chronic progressive disorder characterized by slow onset and acute exacerbations, the design of animal models able to reproduce these characteristics is difficult.
The distribution of these population characteristics is difficult to control in field trials in a commercial setting, emphasizing the need for random allocation to evenly distribute these potential confounders to treatment groups.
Even if this could be feasible, full knowledge of real characteristics is difficult to be obtained and requires information fusion (i.e., official frequency allocation on a per country/region case, applied protocols per band, protocol specifications) from multiple heterogeneous sources.
