This book is designed as an advanced guide for mathematical methods in the science. It covers many topics: classical probability, probability distribution for discrete and continuous random variable, joint probability distribution, expectation value, variance and covariance, Chebishev theorem, discrete distributions: uniform, binomial, multinomial, poisson, geometric, hypergeometric and the binomial negative distribution, continuous uniform distribution: uniform, exponential, gamma, chi-square, normal logarithmic, weibull, normal, central limit theorem, confidence intervals, selection of the right size of a sample, hypothesis testing of a sample, test related with proportions, hypothesis testing for two samples for independent and dependent samples, test of proportion of two samples, Anova test, regression analysis, correlation coefficient, etc. It is included the deduction of many formulas in order to clear the concepts of the Inferential Statistics applied in science . It is hoped that this book fills all needs of the students to get the fundaments of the Inferential Statistics and to achieve the the interest and motivation of the the students for this topic.