The phrase "affect the difficulty of" is correct and usable in written English.
You can use it when discussing how one factor influences the level of challenge associated with a task or situation.
Example: "The complexity of the project can significantly affect the difficulty of meeting the deadline."
Alternatives: "influence the challenge of" or "impact the level of difficulty".
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.
The present study evaluated some factors that could affect the difficulty of aiming and clicking for children from Kindergarten 2 and Grade 1. Adults participated to serve as a reference group.
The distractor word difficulty has also been shown to affect the difficulty of an item (Susanti et al. 2016), so we consider this factor too.
Another study conducted by O'Loughlin and Wigglesworth (2003) have shown that quantity and manner of presentation of information may affect the difficulty of graph-based writing task.
More recently, Loukina et al. (2016) conducted a study to investigate the extent to which textual properties of a text affect the difficulty of listening items in the English test.
The applicability of policies affects the difficulty of implementing policy; the more applicable a policy is, the less difficult it is to implement.
Unlike previous games in the series separate difficulty settings for combat and puzzles are not available, changing the combat difficulty also affects the difficulty of puzzles.
It is possible that the aforementioned "voids" in primary education, either on the level of curricula and textbooks or of the educators themselves, affect the difficulties that exist in the learning of evolution in older ages as well.
It is easy to think of Ebola as an outlier, a rare and exotic disease that is particularly hard to learn about due to the sporadic nature of its outbreaks, the remoteness of the communities it affects, and the difficulty of containing transmission from its animal hosts.
