Management Accounting Consultancy Report


Chapter 3 Cost Behaviour, Cost Drivers and Cost Estimation

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For this topic you should be able to:

Explain the relationship between cost estimation, cost behaviour and cost prediction

Understand the concept of cost drivers, including volume-based and non-volume based cost drivers, and the hierarchy of cost drivers – unit, batch, product, facility

Identify and analyse different cost behaviours, including variable, fixed, step-fixed, semivariable & curvlinear

Use different approaches to cost estimation, including managerial judgement, the engineering approach and quantitative analysis

Explain the difficulties of estimating costs in practice and think of ways to overcome them

The Relationship between Cost Behaviour, Estimation, and Prediction






The process of determining cost behaviour

The relationship between a cost and the level of activity

Using knowledge of cost behaviour to forecast the level of cost at a particular level of activity

How do We Estimate Cost Behaviour?

…by using cost drivers

What is a cost driver?

An activity or factor that drives a cost to be incurred

In choosing appropriate cost drivers, consideration needs to be given about the underlying causes of the costs

What drives it to go up or down?


The cost of paint goes up

Why? What drives the increase?

There is an increase in the number of cars sprayed

Cost driver = number of


A volume-based cost driver

Example 1: A Spray-Painting Business

Administration costs in the corporate headquarters has gone down. Why?

Senior managers decided to hold less face-to-face meetings and communicate electronically instead

Cost driver = number of face-to- face senior management


A non volume-based cost driver

Example 2: A Large Bank

Conventional vs. Contemporary Cost Drivers

Conventional approach:

Uses volume-based cost drivers (e.g., Units produced, DLH, DL$, MH)

Assumes that all costs are driven by the organisation’s level of activity, for example production or sales volume

This will be accurate for direct product costs such as direct material and labour….but not for other types of costs

Contemporary approach:

Recognises that there are a range of possible cost drivers other than production volume that explain cost behaviour

Uses both volume and non-volume based cost drivers

Activity-based Approach to Cost Drivers

Classifies costs and cost drivers into four levels:

Unit level

Batch level

Product (or product-sustaining) level

Facility level

Unit level costs:

Relate to activities performed for each unit produced

Use conventional volume-based cost drivers

Example: Direct material

Batch level costs:

Relate to activities performed for a group of product units

Examples include a batch or a delivery load

Example: Delivery costs

Product level costs:

Also called product sustaining costs

Relates to activities performed for specific products or product groups

Example: Research & design costs

Facility level costs:

Also called facility- sustaining costs

Costs incurred to run the business, not caused by any particular product

Example: Premises costs







Selecting the Best Cost Drivers

How detailed should the analysis be?

As total costs of the organisation are split into smaller cost categories and associated costs drivers, the accuracy of the resulting information will increase

Cost-benefit criteria are important

Long or short term?

Cost behaviour and cost drivers can change over time

Choice depends on the intended purpose of the cost analysis, for example short-term budgeting or long-term strategic decision making?

Selecting the Best Cost Drivers…. (cont’d)

Cost estimation or cost management?

Cost estimation: understand cost

behaviour so that we can estimate cost

functions or predict future costs

Cost management: analysing costs to reduce costs and manage resources more effectively

Effective cost management requires the identification of root cause cost drivers – the basic factors that cause a cost to be incurred

Cost drivers that are used to predict costs may differ from those used to manage costs

The research department sends out blood samples for examination to an external pathology lab.

What is the cost driver of this activity?

For cost estimation purposes: Number of blood samples sent

For cost management purposes: Skill level of staff

Research Cost - $1.5 million – Pharmaceutical Company

Remember last week?!

Cost-benefit Considerations

To predict costs accurately, there is a trade-off between:

Having a strong correlation between the cost and cost driver, therefore increasing the accuracy of the cost information and

Having a cost driver that is easy to measure

Many organisations select cost drivers for convenience reasons, not for their ability to predict costs accurately

Also, with more cost drivers, the costs of gathering and analysing information about costs and cost drivers will also increase

Need to consider cost vs. benefits

A matter of managerial judgement


When choosing cost drivers, the costs and benefits of each driver must be assessed, taking into account:

Reasons for analysing cost behaviour

Timeframe for analysing the cost behaviour

Availability of data on cost drivers

Cost Behaviour Patterns

Cost behaviour

The relationship between a cost and the level of activity (or cost driver)

Cost behaviour patterns

Variable costs

Fixed costs

Step-fixed costs

Semivariable costs

Curvilinear costs

Variable Costs

Variable costs change in total, in direct proportion to changes in the level of activity, but the variable cost per unit remains constant



Fixed Costs

Fixed costs remain unchanged in total despite changes in the level of activity, but fixed cost per unit changes

Fixed cost per unit is often calculated for use in product costs but is of limited use in management decision making as it does not reflect the way that fixed costs actually behave

Modern approaches to cost analysis recognise that there are cost drivers for some of these fixed costs and very few costs actually remain fixed

Activity based costing in Week 7



Constant (compare with variable


Lecture Illustration 1

The following table shows the costs during a month if 600 cars are spray-painted

Required: Fill in the values labelled (a) to (p) in the table

  Number of spray-painted cars
  500 600 700
 Fixed Costs (a) $84 000 (b)
 Variable costs (c) (d) (e)
 Total costs (f) $144 000 (g)
Cost per spray-painted car:
  Fixed cost (h) (i) (j)
  Variable cost (k) (l) (m)
 Total cost per car: (n) (o) (p)

Lecture Illustration 1

The following table shows the costs during a month if 600 cars are spray-painted

Required: Fill in the values labelled (a) to (p) in the table

  Number of spray-painted cars
  500 600 700
 Fixed Costs $84 000
 Variable costs
 Total costs $144 000
Cost per spray-painted car:
  Fixed cost
  Variable cost
 Total cost per car:

Variations in Cost Behaviour Patterns

Step-fixed costs

Remain fixed over a wide range of activity levels but jump to a different amount for levels outside that range

Semi-variable (or mixed) cost

Has both fixed and variable components

Curvilinear cost

At lower levels of activity there is decreasing marginal cost

At higher levels of activity there is increasing marginal cost

Marginal cost: the cost of producing one additional unit

As opening hours increase, more managers are employed

Mixed Costs

Most costs are mixed or semi-variable

This means they have both a fixed and variable cost component

They can be represented using the following cost function:

Y = a + bX

Where Y = total cost

a = fixed cost component (the intercept on the vertical axis)

b = variable cost per unit of activity (the slope of the line)

X = the level of activity

Petrol, oil and maintenance costs – vary proportionately with number of deliveries made

Lease, insurance, and delivery staff salary costs – remain constant irrespective of number of deliveries

For up to 1000 batches, marginal electricity costs decrease as economies of scale are achieved.

Above 1000 batches, older machinery needs to be used so marginal electricity costs increase.

Cost Behaviour and the Relevant Range

The relevant range is the range of activity over which a particular cost behaviour pattern is assumed to be valid

For example:

The direct material cost per unit may only hold for production up to 1000 units per day, and for higher volumes the cost per unit may decrease due to cheaper cost of buying material in larger quantities

Cost Estimation

Identifying the specific cost behaviour pattern for each cost

Approaches to cost estimation:

Managerial judgment

Engineering approach

Quantitative analysis

Visual fit method

High-low method

Regression analysis

Managerial Judgment

Using experience and knowledge rather than formal analysis to classify costs behaviour

Future costs are estimated by examining past costs and identifying other factors that might affect costs in the future

Reliability of cost estimates is dependent upon the ability of the manager

Engineering Approach

Studying processes that result in the incurrence of a cost

Using time and motion studies - employees are observed as they work to record the steps for each task and the times taken

Useful when there is no reliable past data on which to base cost estimates

Most effective when there is a direct relationship between inputs and outputs

However, expensive and time-consuming

Quantitative Analysis

Formal analysis of past data to identify the relationships between costs and activities

Three approaches:

Visual fit method - a scatter plot diagram involves plotting the data points to visualise the relationship between cost and level of activity

High-low method - involves taking the two observations with the highest and lowest level of activity to calculate the cost function

Regression analysis - is a statistical technique that uses a range of data points to estimate the relationship between cost and cost drivers

Example: Dental Clinic

Month No. of patients Material costs
1 160 $3,400
2 120 $3,000
3 150 $3,200
4 100 $2,500
5 80 $2,100
6 130 $3,200
7 140 $3,000
8 190 $4,100
9 230 $5,100
10 170 $3,800
11 160 $3,200
12 180 $3,900


Visual Method - Scatter Plot Diagram

Material costs 160 120 150 100 80 130 140 190 230 170 160 180 3400 3000 3200 2500 2100 3200 3000 4100 5100 3800 3200 3900

No. of patients

Material costs

Lecture Illustration 2: High-Low Method

Nature Clinic’s data for last year in relation to in-house diagnostic blood tests are as follows:

Required: Use the high–low method to estimate the company’s cost behaviour and express it in equation form.

 Month No. of blood tests Cost
January 6,100 $63,500
February 5,300 $50,500
March 4,900 $50,500
April 4,800 $51,500
May 5,100 $51,500
June 3,000 $31,000
July 4,500 $44,500
August 7,100 $72,000
September 6,200 $57,000
October 4,700 $46,500
November 5,900 $64,500
December 6,000 $61,000

Illustration 2: High-Low method Calculations:



Regression Analysis

A statistical technique used to estimate the relationship between a dependent variable (cost) and independent variables (cost driver)

The line of best fit makes deviations between the cost line and the data points as small as possible

More accurate than high-low method as it makes use of all data and has statistical properties that allows inferences to be drawn between cost and activity levels

Simple regression involves estimating the relationship between the dependent variable (Y) and one independent variable (X)

Y = a + bX

Multiple regression estimates a linear relationship between one dependent variable and two or more independent variables

Y = a + b1X1 + b2X2

Regression Statistics
Multiple R 0.973
R Square 0.947
Adjusted R Square 0.942
Standard Error 188.713
Observations 12
  df SS MS F Signif. F
Regression 1 6,366,372 6,366,372 178.767 0.0000
Residual 10 356,128 35,613
Total 11 6,722,500      
  Coefficients Std. Error t Stat P-value Lower 95% Upper 95%
Intercept 561.05 217.40 2.58 0.03 76.65 1045.44
No. of patients 18.66 1.40 13.37 0.00 15.55 21.77

Regression Analysis

Ŷ = $561 + $18.66X where

Y= Material cost

X = No. of patients

Dependent Variable

Independent Variable

R2 - measures the proportion of change in the dependent variable that is explained by a change in the independent variable

Adjusted R2 – an R2 adjusted to compensate for a small sample size

Evaluating the Regression Line

Practical Issues in Cost Estimation Data

Cost estimates are only as good as the data upon which they are based



Issues in








time periods



fixed costs

Data Collection Problems

Missing data

Outliers—extreme observations of activity/cost relationships

Mismatched time periods for dependent and independent variables

Trade-offs in choosing the time period—the number of observations vs the reliability of past data points as predictors of future cost behaviour

Data Collection Problems

Allocation of fixed costs (on a per unit basis) may suggest cost behaviours that are misleading

Inflation may cause historic cost data to be less relevant in predicting future cost behaviours

Effect of learning – labour costs decrease over time

Other Issues in Cost Estimation

Activity-based approaches allow more complex cost behaviour patterns to be considered.

The accuracy of cost functions.

Sometimes budgets and cost estimates capture

only approximations of cost behaviours

All cost functions are based on simplifying assumptions, such as:

Cost behaviours depend on a single or only a few types of activity.

Cost behaviours are linear within a relevant range.

Costs of producing more accurate cost estimates need to be assessed against the likely benefits.




Some slides contained in this presentation were adapted from: