When two phenomena are interrelated, the linkage between them may be characterized as either strong or close. Google suggests that the two are equally common in both academic and non-academic contexts. Are the two completely synonymous, or are there nevertheless differences between them (as slight as they might be) in meaning or linguistic register?
A few examples from Google Scholar: 1. "strong linkage between inequality and political violence" 2. "close linkage between cultural ideals of masculinity and femininity and sexual expression." 3. a strong linkage between cultural values and constitutional choices" 4. "close linkage between social rank and access to resources"