From a statistical point of view, the regression analysis is an area of ongoing research, so an ever-growing collection of techniques makes it difficult the selection of the most adequate one for a given problem.
Choosing which letters to print, from a trawl of several thousand, was more difficult, though my criteria of selection were straightforward.
At any rate, in looking over her early essays "Notes on Camp" and "Against Interpretation," she reacted with contradictory feelings, which made the process of selection difficult.
But proving that natural selection has been at work in very recent times — in this case, the last 300 years — is very difficult, because the traces of selection are still small.
Exact incidences are difficult to obtain because of selection biases, probably leading to substantial underreporting.
Fluctuating selection pressures through time make point estimates of selection difficult to interpret, and consequently, a long-term perspective on selection becomes that much more important.
Hodgson has described it as one of the more difficult selections of his professional life.
Intentional alterations in the nutrient composition of a GM crop can also make the choice of comparator difficult, requiring the selection of an alternative crop in addition to a near-isogenic comparator.
Questions relating to evolvability are also tied up with difficult issues relating to the units-of-selection debate (Okasha 2006).
With only a sample from one timepoint, for example, with the human selection scans described above, it is difficult to obtain quantitative estimates of selection coefficients.
