Question

In: Operations Management

A company has the following history of sales for an item. Month       Sales Volume Jan.             ...

A company has the following history of sales for an item.

Month       Sales Volume

Jan.              120

Feb.                90

Mar.             100

Apr.               75

May             110

Jun.                50

Jul.                 75

Aug.             130

Sep.              110

Oct.              90

Compute the monthly demand forecast for April through November using a 3-month moving average. Compute the monthly demand forecast for June through November using a 5-month moving average. Compute the monthly demand forecast for April through November using a 3-month weighted moving average. Use weights of 0.5, 0.33, and 0.17, with the heavier weights on the more recent months. Compute the mean absolute deviation (MAD) for June through October for each of the methods used. Which method would you use to forecast demand for November?

Solutions

Expert Solution

3-month moving average: It is calculated by the average of the sales volumes of the previous 3 months

So, the 3-month moving average of April = (Sales Volume of Jan + Sales Volume of Feb + Sales Volume of March) / 3

The 3-month moving average from April to November is calculated below:

Month Sales Volume 3-month moving average
Jan 120
Feb 90
Mar 100
Apr 75 103.33
May 110 88.33
Jun 50 95
Jul 75 78.33
Aug 130 78.33
Sep 110 85
Oct 90 105
Nov 110

5-month moving average: It is calculated by the average of the sales volumes of the previous 5 months

So, the 5-month moving average of June = (Sales Volume of Jan + Sales Volume of Feb + Sales Volume of March Sales Volume of April + Sales Volume of May) / 5

The 5-month moving average from June to November is calculated below:

Month Sales Volume 5-month moving average
Jan 120
Feb 90
Mar 100
Apr 75
May 110
Jun 50 99
Jul 75 85
Aug 130 82
Sep 110 88
Oct 90 95
Nov 91

3-month weighted moving average: It is calculated by below formula

The 3-month weighted moving average of April = ((Sales Volume of Jan x Weight of Jan)+ (Sales Volume of Feb x Weight of Feb) + (Sales Volume of March x Weight of March) / (Total Weight)

The weights are 0.5, 0.33 and 0.17, and the highest weight shall be allocated to the most recent months.

So in the above formula, the Weight of March shall be 0.5, Weight of Feb shall be 0.33 and Weight of Jan shall be 0.17.

Also, Total weight = 0.5 + 0.33 + 0.17 = 1

The 3-month weighted moving average from April to November is calculated below:

Month Sales Volume 3-month Weighted moving average
Jan 120
Feb 90
Mar 100
Apr 75 100.1
May 110 85.8
Jun 50 96.75
Jul 75 74.05
Aug 130 72.7
Sep 110 98.25
Oct 90 110.65
Nov 103.4

Mean Absolute Deviation:

Mean Absolute Deviation is calculated as the average value of absolute errors(Sales - Forecast) for all months.

So, for 3-month moving average:

Month Sales Volume 3-month moving average Error Absolute Error
Jan 120
Feb 90
Mar 100
Apr 75 103.33 -28.33 28.33
May 110 88.33 21.67 21.67
Jun 50 95 -45.00 45.00
Jul 75 78.33 -3.33 3.33
Aug 130 78.33 51.67 51.67
Sep 110 85 25.00 25.00
Oct 90 105 -15.00 15.00
Nov 110

MAD = (28.33 + 21.67 + 45 + 3.33 + 51.67 + 25 + 15) / 7 = 190 / 7 = 27.17

For 5-month moving average:

Month Sales Volume 5-month moving average Error Absolute Error
Jan 120
Feb 90
Mar 100
Apr 75
May 110
Jun 50 99 -49 49
Jul 75 85 -10 10
Aug 130 82 48 48
Sep 110 88 22 22
Oct 90 95 -5 5
Nov 91

MAD = ( 49 + 10 + 48 + 22 + 5 ) / 5 = 134 / 5 = 26.8

For 3-month weighted moving average:

Month Sales Volume 3-month Weighted moving average Error Absolute Error
Jan 120
Feb 90
Mar 100
Apr 75 100.1 -25.1 25.1
May 110 85.8 24.2 24.2
Jun 50 96.75 -46.75 46.75
Jul 75 74.05 0.95 0.95
Aug 130 72.7 57.3 57.3
Sep 110 98.25 11.75 11.75
Oct 90 110.65 -20.65 20.65
Nov 103.4

MAD = ( 25.1 + 24.2 + 46.75 + 0.95 + 57.3 + 11.75 + 20.65) / 7 = 186.7 / 7 = 26.67

For forecasting the demand for November, I would use the 3-month weighted average method. This method considers the sales volume for 3 months and also gives more weightage to recent months. The 3-month moving average just considers the average of 3 months and doesn't give any weightage to months. The 5-month moving average considers a broader range ie 5 months, which may not be very accurate to forecast and also doesn't give any weightage to months.

Due to the above reasons, I would use the 3-month weighted average.


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