The invention is a computer-implementable methodology for quantitatively assessing the impact of paid search advertising on the total visits (paid and organic search-based visits) from search engines and utilizing this assessment to improve search engine marketing performance. The methodology relies on the computation of a synergy score for each search engine keyword of interest or coefficients in a synergy equation. Once computed, the score can be used to repeatedly compute the total return on advertising spend (ROAS) and other performance metrics on a go-forward basis without the time lag inherent in computing the synergy score itself. The invention includes specific computer-implementable methods for improving search engine marketing performance based on the total (paid plus organic) performance metrics.