The second characteristic of overhead deals with how different items of overhead change in response to a change in production volume.Overhead can be fixed,variable,or semi variable.As discussed in Chapter 3,fixed overhead remains relatively constant regardless of changes in the level of output,within the relevant range;another way of stating that relationship is that fixed overhead per unit of output varies inversely with production volume.Variable overhead changes proportionately with production volume,within the relevant range;in other words,variable overhead per unit of output is constant.Semi variable overhead is neither fixed nor variable;its amount changes,but not in proportion to production volume.As volume changes,the different overhead cost behavior patterns cause per-unit manufacturing cost to fluctuate considerably.As a result,some method is needed to determine an amount of overhead charged to the units produced.