Probability density function is a statistical expression defining the likelihood of a series of outcomes for a continuous variable, such as a stock or ETF return.

Leonard Fortuin, Five Popular Probability Density Functions: A Comparison in the Field of Stock-Control Models, The Journal of the Operational Research Society, Vol. 31, No. 10 (Oct., 1980), pp.

The problem of estimating a probability density function has only recently begun to receive attention in the literature. Several authors [Rosenblatt (1956), Whittle (1958), Parzen (1962), and Watson ...