APPENDIX 8C. LIFETIME DISTRIBUTIONS 

TABLE OF CONTENTS 

8C.1 INTRODUCTION ...................................................................................................8C-1 

8C.2 DERIVATION OF WEIBULL DISTRIBUTION PARAMETERS .......................8C-1 

8C.3 COOKING PRODUCTS – COOKTOP AND OVEN LIFETIME 

DISTRIBUTIONS ...................................................................................................8C-2 

8C.4 MICROWAVE OVEN LIFETIME DISTRIBUTIONS..........................................8C-5 

LIST OF TABLES 

Table 8C.3.1 Cooktops and Ovens ....................................................................................8C-2 

Table 8C.4.1 Microwave Ovens ........................................................................................8C-5 

LIST OF FIGURE 

Figure 8C.3.1 Fraction of the Electric Cooktops and Ovens Failing..................................8C-3 

Figure 8C.3.2 Cumulative Lifetime Length of Electric Cooktops and Ovens....................8C-3 

Figure 8C.3.3 Fraction of the Gas Cooktops and Ovens Failing ........................................8C-4 

Figure 8C.3.4 Cumulative Lifetime Length of Gas Cooktops and Ovens..........................8C-4 

Figure 8C.4.1 Fraction of the Microwave Ovens Failing ...................................................8C-5 

Figure 8C.4.2 Cumulative Lifetime Length of Microwave Ovens.....................................8C-6 

8C-i 

EERE-2006-STD-0127 COMMENT 64.2
APPENDIX 8C. LIFETIME DISTRIUBTIONS 

8C.1 INTRODUCTION 

For each product class, DOE characterized the product lifetime using a Weibull 

probability distribution that ranged from the minimum to maximum lifetime estimates described 

in chapter 8, section 8.2.3. The Weibull distribution is recommended for applying lifetime data 

because it can be shaped to match low, most likely or average, and high values, yet still allow 

some probability of exceeding the high value. The Weibull distribution has a long tail, which the 

analyst can use to specify a distribution with some chance of exceeding the high value.1,2 

8C.2 DERIVATION OF WEIBULL DISTRIBUTION PARAMETERS 

DOE utilized an approach for calculating the Weibull distribution using product lifetime 

data to determine low, average, high value, and percentile of high value. A similar approach is 

described by Crystal Ball Technical Note, which uses most likely value instead of average.3 

Available data is used to assign low, average, and high values to a random variable with 

unknown distribution parameters. 

 The Weibull distribution can be defined as: 

Eq. 1 

where 

L = location 

Alpha = scale 

Beta = shape 

Therefore, the cumulative distribution is 

Eq. 2 

Based on available data, Weibull distribution parameters are specified in the following 

manner: 

1. The output deviates must be greater than the expert opinion for low value, 

2. The average Xavg, must be equal to the average value from the available data, 

3. The high value, xb, must correspond to some particular percentile point (e. g., 95%, or 90%) 

8C-1

EERE-2006-STD-0127 COMMENT 64.2
The values for the parameters in Eq.1 and Eq. 2 are determined using the approach 

outlined in Crystal Ball’s Technote.3 

This solution can be checked using Crystal Ball by specifying a Weibull distribution with 

the calculated parameters (location, scale, and shape) in an assumption cell and generate a 

forecast that equals the assumption. Forecast histogram and statistics verify that the Weibull 

distribution matches the desired shape. 

8C.3 COOKING PRODUCTS – COOKTOP AND OVEN LIFETIME 

DISTRIBUTIONS 

Table 8C.3.1 shows the average, minimum, maximum lifetime, and maximum percentile 

values used to determine the Weibull distribution parameters alpha and beta. DOE estimated 

that the maximum lifetime percentile for both electric and gas cooking products was 99 percent. 

Table 8C.3.1 Cooktops and Ovens 

Expert Opinion Values 

Weibull 

Parameters 

Product Class 

Minimum 

years 

Average 

years 

Maximum 

years 

Maximum 

percentile 

% 

Alpha 

(scale) 

Beta 

(shape) 

Electric Cooktops and Ovens 10 19 28 99 10.126 2.655 

Gas Cooktops and Ovens 12 19 26 99 7.876 2.655 

Figures 8C.3.1 to 8C.3.2 show the Weibull distribution as well as the cumulative Weibull 

distribution for electric cooktops and ovens. Figures 8C.3.3 to 8C.3.4 show the Weibull 

distribution as well as the cumulative Weibull distribution for gas cooktops and ovens. 

8C-2

EERE-2006-STD-0127 COMMENT 64.2
Probability of Failure (%) 

11%

10%

9%

8%

7%

6%

5%

4%

3%

2%

1%

0%

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34

Age of Equipment (Years) 

Figure 8C.3.1 Fraction of the Electric Cooktops and Ovens Failing 

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34

Age of Equipment (Years) 

Probability (%) 

Figure 8C.3.2 Cumulative Lifetime Length of Electric Cooktops and 

Ovens 

8C-3 

EERE-2006-STD-0127 COMMENT 64.2
Probability of Failure (%) 

15%

10%

5%

0%

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30

Age of Equipment (Years) 

Figure 8C.3.3 Fraction of the Gas Cooktops and Ovens Failing 

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30

Age of Equipment (Years) 

Probability (%) 

Figure 8C.3.4 Cumulative Lifetime Length of Gas Cooktops and 

Ovens 

8C-4 

EERE-2006-STD-0127 COMMENT 64.2
8C.4 MICROWAVE OVEN LIFETIME DISTRIBUTIONS 

Table 8C.4.1 shows the average, minimum, maximum lifetime, and maximum percentile 

values used to determine the Weibull distribution parameters alpha and beta. DOE estimated 

that the maximum lifetime percentile for microwave ovens was 99 percent. 

Table 8C.4.1 Microwave Ovens 

Product Class 

Expert Opinion Values 

Weibull 

Parameters 

Minimum 

years 

Average 

years 

Maximum 

years 

Maximum 

percentile 

% 

Alpha 

(scale) 

Beta 

(shape) 

Gas-Fired Pool Heaters 7 9 10 99 2.183 4.806 Probability of Failure (%) 

Figures 8C.4.1 to 8C.4.2 show the Weibull distribution as well as the cumulative Weibull 

distribution for microwave ovens. 

90%

80%

70%

60%

50%

40%

30%

20%

10%

0%

0 1 2 3 4 5 6 7 8 9 1 0 11

Age of Equipment (Years) 

Figure 8C.4.1 Fraction of the Microwave Ovens Failing 

8C-5

EERE-2006-STD-0127 COMMENT 64.2
0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

0 1 2 3 4 5 6 7 8 9 1 0 11

Age of Equipment (Years) 

Probability (%) 

Figure 8C.4.2 Cumulative Lifetime Length of Microwave Ovens 

8C-6

EERE-2006-STD-0127 COMMENT 64.2
REFERENCES 

1 Barnes, P.R., J. W. Van Dyke, B. W. McConnell, S. M. Cohn, and S. L. Purucker, The 

Feasibility of Replacing or Upgrading Utility Distribution Transformers During Routine 

Maintenance. 1995, Oak Ridge National Laboratory: Oak Ridge, TN. 

2 Karr, T., Making the Most of Life Test Data, in Appliance Magazine. 2003. 

3 Crystal Ball. Technote: Derivation of Weibull Distribution Parameters When the Minimum, 

Most Likely Value, and a Percentile are Known. 2007 [cited; Available from: 

http://www.crystalball.com/support/simulation/cbl_gen_021A.html. 

8C-7

EERE-2006-STD-0127 COMMENT 64.2