Cost * and activity level
　　1 The basic principle of cost * is that as the level of activity rises,costs will usually rise,It will cost more to produce 2,000 units of output than it will cost to produce 1,000 units.
　　2 This principle is common sense,The problem for the accountant,however,is to determine,for each item of cost,the way in which costs rise and by how much as the level of activity increases,For our pruposes here,the level of activity for measuring cost will generally be taken to be the volume of production,
　　【Example】Hans Bratch has a fleet of company cars for sales representatives.Running costs have been estimated as follows.
　　a) Cars cost $12,000 when new,and have a guaranteed trade-in value of $6,000 at the end of two years.Depreciation is charged on a straight-line basis.
　　b) Petrol and oil cost 15 cents per mile.
　　c) Tyres cost $300 per set to replace;replacement occurs after 30,000 miles.
　　d) Routine maintenance costs $200 per car (on average)in the first year and $450 in the second year.
　　e) Repairs average $400 per car over two years and are thought to vary with mileage.The average car travels 25,000 miles per annum.
　　f) Tax,insurance,membership of motoring organisations and so on cost $400 per annum per car.
　　Required
　　Calculate the average cost per annum of cars which travel 15,000 miles per annum and 30,000 miles per annum.
　　Solution
　　Costs may be analysed into fixed,variable and stepped cost items,a stepped cost being a cost which is fixed in nature but only within certain levels of activity.
　　(1)
　　Fixed costs                                      $ per annum
　　Depreciation $(12,000－6,000) ÷2                    3,000
　　Routine maintenance $(200＋450) ÷2                   325
　　Tax,insurance etc                                     400
　　3,725
　　(2)
　　Variable costs                                    Cents per mile
　　Petrol and oil                                        15.0
　　Repairs($400÷50,000 miles)*                           0.8
　　15.8
　　*If the average car travels 25,000 miles per annum,it will be expected to travel 50,000 miles over two years (this will correspond with the repair bill of $400 over two years).
　　(3)
　　Step costs are tyre replacement costs,which are $300 at the end of every 30,000 miles.
　　1) If the car travels less than or exactly 30,000 miles in two years,the tyres will not be changed,Average cost of tyres per annum＝$0.
　　2) If a car travels more than 30,000 miles and up to (and including)60,000 miles in two years,there will be one change of tyres in the period.Average cost of tyres per annum＝$150($300÷2).
　　3) If a car exceeds 60,000 miles in two yars (up to 90,000 miles)there will be two tyre changes,Average cost of tyres per annum＝$300($600÷2).
　　The estimated costs per annum of cars travelling 15,000 miles per annum and 30,000 miles per annum would therefore be as follows.
　　15,000 miles       30,000 miles
　　per annum$        per annum$
　　Fixed costs                     3,725              3,725
　　Variable costs(15.8c per mile)  2,370              4,740
　　Tyres                            ﹣                150
　　Cost per annum                  6,095              8,615