I find it interesting that CCNA Voice (one additional exam after achieving CCNA) has a base salary mean of over $97K. CCNP Voice requires 4 additional exams after CCNA Voice, and is a much more difficult certification. Since CCNA Voice is a requisite for CCNP Voice, I wonder if there is a false perception that CCNA-V alone is worth $97K, and whether most of the respondents with CCNA-V and higher salaries also have achieved CCNP-V certification. It's equivalent to stating that people with high school diplomas have a base salary mean of $X amount, when most of those with the higher salaries that do have their high school diplomas also have their doctorates - which is what really brings up the mean (and average).

I think a simple way to show how one particular certification affects mean and average salary is to exclude any lower related certifications from the person's response. A complicated but interesting way would be to display dynamic charts that show all certifications for each respondent, something like a TED presentation.