In some skills and crafts there will be no actual displayed creation chance. [[Mining]] to cite one example.
'''Improving''' - Having the best (high QL) tools and raw materials (resources) you can get your claws on is best. In addition, finding the one item in the skill that uses the least amount of raw materials for improving is desirable.
'''Creation''' - Having a creation chance around 50% seems to give the best skill gain. This chance is calculated based on a number of factors to varying degrees: skill, raw material ql, tool ql, and etc. The exact emphasis can differ from item to item. Typically the quality of the parts and tools have the most impact with skill playing a close second.