The phrase "a more difficult set" is correct and usable in written English.
It can be used when comparing the difficulty level of a particular set of items, tasks, or problems to another set.
Example: "After reviewing the previous exercises, I realized that this is a more difficult set for the students to tackle."
Alternatives: "a tougher set" or "a harder set".
If instead, as we all hope, we start returning to growth late this year or early next, ministers will face a more difficult set of choices.
In some ways, because Liberty's operations were focused so heavily in the all-but-deserted eastern half of the city, Mr. McDonald must confront a more difficult set of choices than most of his corporate peers.
Mutually advantageous transactions present a more difficult set of problems.
More able respondents received a more difficult set of items than less able respondents.
But they discussed these hopes in a more difficult setting.
The recently released PC shooter game Soldier of Fortune offers the best of both worlds: an easy setting, for unlimited saves at any time in a level, or a more difficult setting that limits the number of saves per level.
As compared with these studies, the present investigation was performed on a larger set of samples, a broader range of agents and in a more difficult setting, comprising sampling of small children at several health care units in a developing country.
For the more difficult set of concepts, chemical bonding, creating a visual explanation led to much larger gains than creating a verbal one for students both high and low in spatial ability.
Contraction has held up the start of bargaining for a new labor agreement and threatens to make those negotiations more difficult, setting a discordant tone for the talks.
Hence, the National Geographic data set can be seen as a more difficult data set.
Based on McCarley & Krebs (2000) and Krebs & Sinai (2002), we had assumed that a more difficult stimulus set (i.e., degraded quality of image and type of psychophysical task) would lead to higher capacity coefficient values for the algorithmically fused imagery.
