In this technical note, we examine the concept of diversification and the tradeoff between risk and return in portfolio theory. The note includes a general introduction to normal distribution, then applies it to portfolio theory by examining normal distribution of future return and risk. The note also examines formulas for calculating expected returns of various two-asset portfolios and applies these formulas to investment decision-making. Finally, it touches on errors of inference, including survivorship bias and statistical concerns around skewness or excess kurtosis.