The document discusses methods for determining how costs behave and estimating cost functions. It explains that total costs can often be explained by changes in a single activity level and that cost behavior is commonly approximated by linear functions within a relevant range. It then outlines different types of linear cost functions and approaches to estimating cost functions, including the industrial engineering method, conference method, account analysis, and quantitative regression analysis. Key steps in estimation include selecting a dependent cost variable and driver, collecting data, plotting relationships, and evaluating estimated functions.