Strength changes the target character's Strength score, the amount by which is dependent on the character's score before the spell was cast.
Application of this spell increases the Strength of the character to 18 and then adds 50% bonus on top of this. The character receives any Strength bonuses appropriate. If a character has between 18/50 and 18/00 Strength, then his Strength will just be set to 18/00. If a character has a Strength between 18/01 and 18/50, then his Strength will become 18/50 + original percentage. For example, if the character has a Strength of 18/25, this spell will increase his Strength to 18/75. If his Strength is less than 18, then it will just become 18/50. However, this spell will actually lower a character's Strength if it is already 19 or more.