This research is an analysis of the range of skills and knowledge required to produce, effectively, results in the Art of Lyric Improvisation in the field of jazz singing. Lyric Improvisation is the art of retaining the primary lyrics of a song but, using improvisational inventiveness, changing every other aspect. The study focuses on the manipulation of melody, rhythm, time feel, style, range, articulation and improvisation in the performances of renowned jazz vocalists Sarah Vaughan and Carmen McRae. The research is based on their multiple recordings of "Sometimes I'm Happy" (Youmans/Caesar) between the years1955-1965. The method compares different elements of the individual singers' improvisations to the published notation. These elements include: syncopation of the rhythm, motific development, expanding the range of pitch, variances in timbre and articulation and spontaneous re-composition of the melody all while maintaining the original lyric. The outcomes, however, can be applied over a multitude of tunes from any American Song Book composer or jazz standard sung in historically swing styles of the past century, or with contemporary developments.