Which of the following can lead to an uneven curl pattern?

The reason using rods of different sizes can lead to an uneven curl pattern is that the size of the rod directly influences the size and consistency of the curls created during the perming process. When rods of various sizes are used, it results in inconsistencies, as each section of hair wrapped around a different sized rod will have varying amounts of tension and set differently. The smaller rods create tighter curls, while larger rods produce looser curls. This disparity can lead to an unbalanced look in the overall curl pattern, making it crucial to use rods of uniform size for a more harmonious result.

While the other factors mentioned can certainly affect the perm process—such as the condition of the hair when perming or the importance of sectioning for even application—using rods of different sizes is particularly significant in creating uneven curls. This highlights the importance of uniformity in the tools used during hair treatments to achieve the desired aesthetic.