In: Operations Management
Problem 13: Sam's Cat Hotel operates 52 weeks per year, 6 days per week, and uses a continuous review inventory system. It purchases kitty litter for $11.70 per bag. The following information is available about these bags.
Demand = 90 bags/week
Order Cost= $54/0rder
Annual Holding cost = 27% of cost
Desired cycle-service level = 80%
Lead time = 3 weeks (18 working days)
Standard deviation of weekly demand = 15 bags
Current on-hand inventory is 320 bags, with no open orders or backorders.
Suppose that Sam's Cat Hotel in Problem 13 uses a P system instead of a Q system. The average weekly demand is d = 90 bags and the standard deviation of the weekly demand is σ= 15 bags.
a. What P (in weeks) and T should be used to approximate the cost trade-offs of the EOQ?
b. How much more safety stock is needed than with a Q system?
c. It is time for the periodic review. How much kitty litter should be ordered?"
[ Note for students : no rounding is done as no such instruction is
mentioned in the question]
a) EOQ = sqrt((2*annual demand*ordering cost)/holding cost per unit per year) = sqrt((2*90*52*54)/(27%*11.7)) = 400
P = time between 2 orders in weeks = Total number of weeks available/(Annual demand/EOQ) = 52/((90*52)/400) = 4.444444444
T = target inventory = Weekly Demand*(lead time in weeks+P) + z*standard deviation of weekly demand*sqrt(lead time in weeks+P)
=90*(3+4.444444444)+normsinv(0.8)*15*sqrt(3+4.444444444) = 704.4448334 [ Answer is 704 if rounding is required]
b)
Safety stock with Q system => z*standard deviation of weekly demand*sqrt(lead time in weeks) = normsinv(0.8)*15*sqrt(3) = 21.86596106
Safety stock with P system => z*standard deviation of weekly demand*sqrt(lead time in weeks+P) = normsinv(0.8)*15*sqrt(3+4.444444444) = 34.44483348
P system has 34.44483348-21.86596106 = 12.57887242 safety stock more than Q system [ Answer is 13 if rounding is required]
c)
Amount of litter Kitty should order = target inventory-on hand inventory = 704.4448334-320 = 384.4448334 [Answer is 384 if rounding is required]