There is no guessing "penalty" on the AP test. There is also no guessing "reward". It is simply neutral. To understand this imagine a 5 question test. Pure random guessing should result in one correct answer and four incorrect (a simple probability problem). The score would be positive one for the correct answer and negative 1/4 for each incorrect. The net score is zero, the same as leaving all questions blank. I tell my students there is no point in random guessing but if they have any reason to eliminate even one choice they should guess one of the remaining ones. Some students are such perfectionists they are afraid to answer unless they are certain of a correct answer. This definitely decreases their chances of passing. Random guessing on average will not lower scores. Educated guessing will raise scores.