A witness who is qualified as an expert by knowledge, skill, experience, training, or education may testify in the form of an opinion or otherwise if:
(a) the expert’s scientific, technical, or other specialized knowledge will help the trier of fact to understand the evidence or to determine a fact in issue;
(b) the testimony is based on sufficient facts or data;
(c) the testimony is the product of reliable principles and methods; and
(d) the expert has reliably applied the principles and methods to the facts of the case.
Notes
(Pub. L. 93–595, §1, Jan. 2, 1975, 88 Stat. 1937; Apr. 17, 2000, eff. Dec. 1, 2000; Apr. 26, 2011, eff. Dec. 1, 2011.)
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https://www.law.cornell.edu/rules/fre/rule_702
Testimony
https://www.merriam-webster.com/dictionary/testimony
Firsthand
https://www.merriam-webster.com/dictionary/firsthand
Experience
https://www.merriam-webster.com/dictionary/experience
Specialized
https://www.merriam-webster.com/dictionary/specialized
Specialization
1: characterized by or exhibiting biological specialization
especially : highly differentiated especially in a particular direction or for a particular end
2: designed, trained, or fitted for one particular purpose or occupation
https://www.merriam-webster.com/dictionary/specialization
Case
2a : an outer covering or housing
https://www.merriam-webster.com/dictionary/case
Knowledge
https://www.merriam-webster.com/dictionary/knowledge