The safety stock is to prevent unpredictable fluctuations in demand or supply. The minimum stock should always be kept as the safety stock in the warehouse.
influence factor
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(1) Changes in inventory demanddelivery lead time And the length of delivery delay.The greater the change of expected inventory demand, the greater the safety stock that the enterprise should maintain;Similarly, under the same conditions of other factors, the greater the uncertainty of ordering interval and lead time, or the longer the expected ordering interval, the higher the risk of inventory interruption and the higher the safety stock.
(2) Shortage cost and storage cost of inventory.Generally, the higher the probability or possible amount of inventory shortage cost, the greater the safety stock that the enterprise needs to maintain.Increasing the amount of safety stock can reduce the cost of inventory shortage, but it will bring additional burden on the storage cost of enterprises.Under ideal conditions, the optimal ordering and storage mode can be obtained, but in the actual operation process, the ordering cost and storage cost change inversely, and the risk brought by uncertainty makes this problem that has arisen since the commodity circulation has not been effectively solved.
Generally, manufacturers have to deal with two kinds of flows: logistics and information flow.Internal barriers affect the effective flow of information. Batch processing of information makes the "acceleration principle" in the company effective. Demand information is often distorted or delayed, which causes a typical reaction of purchasing personnel and production planners - "lead time or security inventory syndrome".This effect continues to strengthen until the increase is excessive, and the corresponding costs rise at the same time.
The excess production capacity continues to spread to the whole supply chain, and the distorted demand data starts to cause the second effect - "inventory reduction syndrome". Manufacturers have to choose to permanently reduce the sales price of their products, eroding their profits.The former effect causes excessive inventory, and the company does not change the process in order to find a way out and lead to the latter result. The two effects will continue to exist and promote each other.
In the growth period of the market, the consequences of the combination of the two effects are often masked by the growing demand. Manufacturers can survive or even prosper without considering the existence of the shock cycle - for a period of time, fully deal with inventory;In another period of time, the production was accelerated regardless of the cost.When the market entered a period of steady development or decline, manufacturers began to decline step by step.It can be said that when there is a gap between enterprises and even between the internal doors of enterprises, the lag in information transmission, slow response, batch processing and uncertainty are the deep reasons for the above two effects. The root of the response is also to reduce organizational barriers, strengthen information facilitation and be able to respond quickly.
calculation
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The size of safety stock is mainly determined by customer service level (or order satisfaction).The so-called customer service level refers to the degree of satisfaction of customer needs. The formula is as follows:
Customer service level (5%)=annual shortage times/annual order times
The higher the customer service level (or order satisfaction rate) is, the less out of stock occurs, so the cost of out of stock is small. However, the increase of safety stock leads to the increase of inventory holding cost;The low level of customer service indicates that there are many cases of stock outs, high cost of stock outs, and low level of safety stock,Inventory holding costSmaller.Therefore, we must consider the relationship among customer service level, out of stock cost and inventory holding cost, and finally determine a reasonable amount of safety stock.
For the calculation of safety stock, with the help of the knowledge of quantity statistics, the changes of customer demand and lead time are taken as some basic assumptions, so that when customer demand changes, lead time changes and both changes at the same time, the respective safety stock can be calculated.
When the demand changes and the lead time is a fixed constant
Assume that the change of demand meetsNormal distributionBecause the lead time is a fixed value, we can directly calculate the demand distribution in the lead timemean valueAnd standard deviation.Or we can determine the expected mean value and standard deviation of demand based on the demand in the past lead time through direct expectation prediction.The advantage of this method is that it is easy to understand.
Once the mean value and standard deviation of demand conditions within the lead time are determined, the safety stock SS can be obtained by using the following formula.
SS=Z* SQRT(L) * STD
Among them: STD --- within the lead timeStandard variance；
L --- length of lead time;
Z --- the safety factor demanded by a certain customer service level
Example:
The average daily demand for beer in a hotel is 10 gallons, and the beer demand follows the normal distribution with the standard deviation of 2 gallons/day. If the lead time is a fixed constant of 6 days, what is the size of the safety stock that meets 95% of customer satisfaction?
Answer: From the question:
STD=2 gallon/day, L=6 days, F (Z)=95%, then Z=1.65,
That is, when 95% of customers are satisfied, the safety stock is 8.08 gallons.
When the lead time changes and the demand is a fixed constant
If the customer demand within the lead time is a fixed constant, and the length of the lead time is random, in this case: SS is
SS=Z* STD2 * d
Where: STD2 - standard deviation of lead time;
Z ---- the safety factor demanded by a certain customer service level;
D ---- daily demand within the lead time;
Example:
In the above example, if the daily demand for beer is a fixed constant of 10 gallons, the lead time is random, and the average service is 6 daysStandard varianceIf the normal score is 1.5, try to determine the safety stock under 95% customer satisfaction.
Solution: According to the question:=1.5 days, d=10 gallons/day, F (Z)=95%, then Z=1.65,
Thus: SS=Z * STD2 * d=1.65 * 10. * 1.5=24.75
That is, when 95% of customers are satisfied, the safety stock is 24.75 gallons.
3. The demand and lead time are random
In most cases, lead time and demand are random. At this time, we assume that customer demand and lead time are mutually independent, then SS is
SS=Z * SQRT(STD*STD*L + STD2*STD2*D*D)
Where: Z - safety coefficient under certain customer service level;
STD2 - Standard deviation of lead time;
STD -- In the lead timeStandard variance；
D --- Average daily demand within the lead time;
L -- Average lead time level;
Example:
If in the above example, daily demand and lead time are independent of each other, and their changes strictly meetNormal distributionThe daily demand meets the normal distribution with an average of 10 gallons and a standard variance of 2 gallons, and the lead time meets the normal distribution with an average of 6 days and a standard variance of 1.5 days. Try to determine the safety stock under 95% customer satisfaction.
Solution: According to the question, STD=2 gallons, STD2=1.5 days, D=10 gallons/day, L=6 days, F (Z)=95%, then Z=1.65, so: SS=1.65 * SQRT (2 * 2 * 6+1.5 * 1.5 * 10 * 10)=26.04
That is, when 95% of customers are satisfied, the safety stock is 26.04 gallons
