SCC Seminar on Data Analysis-Intro to Zeros
From Daniel Hall
Data with zeros are common in a wide variety of fields including agriculture, biology, medicine, public health, climatology, ecology, and more. Examples include responses that are subject to a limit of detection (e.g., concentrations of pollutants in soil, water, or air); counts of the number of plants or shoots or fruits in an agricultural experiment; proportions of plants, animals, pine cones surviving to maturity, affected by disease, etc.; number of insects of a given species trapped or killed under different conditions; proportion of leaf or fruit surface affected by disease; medical expenditures; rainfall totals; hospital lengths of stay; motor vehicle fatalities; and on and on . Such data present special challenges and are typically not modeled adequately using traditional linear and generalized linear models. Specialized models and techniques are often needed, such as zero-inflated regression models, hurdle models, two-part models, censored regression models, and other methods. In this talk I will give examples of data with excess zeros and I will provide an overview of statistical methodology for handling such data with an emphasis on practical matters.