¶10,201 Common Types of Cost Behavior Fixed costsfundamentally are not driven by changes in the volume of activity. Variable costschange directly and proportionately with the volume of activity. Mixed(or semivariable) costs are costs or cost pools that contain botha fixed and a variable component. Semivariable costschange with the volume of activity, but not proportionately with activity changes. Costs of this nature are often called learning curve costsbecause they increase at a decreasing rate with the volume of activity. Semifixed costsincrease in steps or jumps. Some argue that all fixed costs are really semifixed costs at various levels of activities. ¶10,211 Cost Behavior Assumptions Basis of Cost Behavior Estimates.Two basic assumptions underlie the concept of cost behavior. They are the relevant range assumptions and the time assumption. Although it is convenient to think of costs as being fixed, variable, mixed, or something else, few costs exhibit the same cost behavior pattern for all levels of activity. Instead, costs exhibit a particular cost pattern for a specified range of activity called the relevantrange. Cost behavior estimates are usually based on historical cost observations and analyses. A second important cost behavior assumption is the time assumption, which means that as time passes, the business environment changes, and cost behavior may change as well. Many factors can cause a cost to change patterns over time. Ways of Estimating Cost Behavior.There are several ways of estimating cost behavior: account analysis method, high-low method, regression analysis, and engineering or work-measurement method. Account Analysis Method.The account analysis method classifies the cost accounts in the subsidiary ledger as fixed, variable, etc., based upon experience and judgment. This method is cheap, but subjective. High-Low Method.The high-low method is the simplest quantitative analysis based upon the basic formula y = a + bx, where b is the unit variable cost per measure of activity.