There may be instances where the lay person's testimony would be difficult to understand for any number of reasons, but the opinion would help to clarify the testimony for the court. The opinion must therefore be directly related to the case. Superfluous opinion evidence is inadmissible; it must provide genuine value to the court's understanding of the case. In South Carolina, the added c) clause implies that in addition to the above, the situation must be such that there is no expert witness who could otherwise provide the opinion in question. The only time, then, in South Carolina where lay persons may render opinion is in situations where any reasonable lay person could form such opinions. If the situation is such that the opinion cannot reasonably be formed except by an expert, the lay opinion is inadmissible. This essentially rules out the potential for a lay witness to contradict an expert witness,...

By excluding lay witness testimony in matters requiring an expert witness, the supremacy of expert witness testimony is firmly established in the statute.
Overall, the statutes of the two Carolinas and the federal statutes are almost identical. However, South Carolina makes the added stipulation that rules out conflict between expert and lay witness opinions by rendering the latter inadmissible in situations where the former is available.

Works Cited:

Federal Rules of Evidence retrieved April 7, 2009 from http://www.ilnd.uscourts.gov/LEGAL/FREv/Fre00041.htm

South Carolina Rules of Evidence retrieved April 7, 2009 from http://www.xprolegal.com/rules/Rules_SCarolina.pdf

North Carolina Rules of Evidence retrieved April 7, 2009 from http://www.ncga.state.nc.us/EnactedLegislation/Statutes/PDF/ByArticle/Chapter_8C/Article_7.pdf