Fill rate is the percentage of customer orders that are fulfilled without running out of inventory or placing backorders1. Fill rate is an important measure of customer service and inventory management efficiency. A high fill rate indicates that the company can meet customer demand in a timely and accurate manner, while a low fill rate suggests that the company is struggling to satisfy customer expectations.

Operating costs are the expenses associated with running a business, such as rent, utilities, wages, transportation, etc2. Operating costs are influenced by various factors, such as production volume, inventory level, technology, and quality. A high operating cost means that the company spends more money to produce and deliver its products or services, while a low operating cost means that the company spends less money to do so.

Service level is the measure of how well a company delivers its products or services to its customers, based on criteria such as availability, timeliness, quality, and satisfaction3. Service level is affected by various factors, such as demand variability, supply reliability, capacity utilization, and customer feedback. A high service level means that the company meets or exceeds customer expectations, while a low service level means that the company fails or falls short of customer expectations.

As the company continues its initiative to increase the fill rate on these parts to 100%, it is most likely that operating costs will increase faster than service level. This is because increasing the fill rate requires increasing the inventory level, which in turn increases the carrying costs, such as warehousing, insurance, taxes, and obsolescence4. Moreover, increasing the fill rate also requires reducing the variability and uncertainty in demand and supply, which may involve investing in more advanced technology, improving quality control, enhancing supplier relationships, or implementing demand management techniques5. These actions can also increase the operating costs of the company.

However, increasing the fill rate does not necessarily increase the service level at the same rate. This is because service level depends not only on fill rate, but also on other factors, such as delivery speed, order accuracy, product quality, and customer satisfaction6. Therefore, increasing the fill rate may not be enough to improve the service level significantly. In fact, there may be a point of diminishing returns, where increasing the fill rate beyond a certain level does not result in a proportional increase in service level. For example, increasing the fill rate from 95% to 99% may have a noticeable impact on service level, but increasing it from 99% to 100% may have a negligible impact on service level.