The role of an expert witness is to use “scientific, technical, or other specialized knowledge” to assist the trier of fact to “understand the evidence or to determine a fact in issue.” The “trier of fact” is the jury in a jury trial and the judge in a bench trial. The rationale for Federal Rule of Evidence 702 is that the jury may not be able to understand or correctly decide a case without assistance from an expert witness.
Experts are qualified to testify by their “knowledge, skill, experience, training, or education.”
Note the conjunction “or.” Under Rule 702, the expert only needs to have “knowledge”, “skill”, ”experience”, “training”, OR “education”. She need NOT have all five.