This is my fourth lecture notes about biostatistic, the previous notes, told us about the data presentation ( graphs, polygon, bar chart, histogram and etc. Probability topic will discuss about what happen when we roll a die or tossing a coin, in my opinion this topic will use to help us when we estimate the real distribution in population using several sample. the professor said that this topic is useful to expect the probability when we doing research in large population and doesn’t know about the real population distribution.