KEY PERFORMANCE INDICATORS
Below is a summary of some of the quantitative performance indicators (as defined below) that are evaluated by management to assess our financial performance. Some of the indicators are non-GAAP measures and should not be considered as an alternative to any other measure for determining operating performance or liquidity that is calculated in accordance with generally accepted accounting principles.
Definitions and analysis of these performance indicators are as follows:
Net Sales Net sales include sales of lasers, laser systems, related accessories and service. Net sales for fiscal 2019 decreased 29.6% in our OLS segment and decreased 15.4% in our ILS segment from fiscal 2018. For a description of the reasons for changes in net sales refer to the ‘‘Results of Operations’’ section below.
Gross Profit as a Percentage of Net Sales Gross profit as a percentage of net sales (‘‘gross profit percentage’’) is calculated as gross profit for the period divided by net sales for the period. Gross profit percentage for OLS decreased to 47.3% in fiscal 2019 from 52.7% in fiscal 2018. Gross profit percentage for ILS decreased to 13.3% in fiscal 2019 from 26.7% in fiscal 2018. For a description of the reasons for changes in gross profit refer to the ‘‘Results of Operations’’ section below.
Research and Development as a Percentage of Net Sales Research and development as a percentage of net sales (‘‘R&D percentage’’) is calculated as research and development expense for the period divided by net sales for the period. Management considers R&D percentage to be an important indicator in managing our business as investing in new technologies is a key to future growth. R&D percentage increased to 8.2% in fiscal 2019 from 7.0% in fiscal 2018. For a description of the reasons for changes in R&D spending refer to the ‘‘Results of Operations’’ section below.
Net Cash Provided by Operating Activities Net cash provided by operating activities shown on our Consolidated Statements of Cash Flows primarily represents the excess of cash collected from billings to our customers and other receipts over cash paid to our vendors for expenses and inventory purchases to run our business. We believe that cash flows from operations is an important performance indicator because cash generation over the long term is essential to maintaining a healthy business and providing funds to help fuel growth. For a description of the reasons for changes in Net Cash Provided by Operating Activities refer to the ‘‘Liquidity and Capital Resources’’ section below.
Days Sales Outstanding in Receivables We calculate days sales outstanding (‘‘DSO’’) in receivables as net receivables at the end of the period divided by net sales during the period and then multiplied by the number of days in the period, using 360 days for years. DSO in receivables indicates how well we are managing our collection of receivables, with lower DSO in receivables resulting in higher working capital availability. The more money we have tied up in receivables, the less money we have available for research and development, acquisitions, expansion, marketing and other activities to grow our business. Our DSO in receivables for fiscal 2019 remained unchanged at 67 days as compared to fiscal 2018.
Annualized Fourth Quarter Inventory Turns We calculate annualized fourth quarter inventory turns as cost of sales during the fourth quarter annualized and divided by net inventories at the end of the fourth quarter. This indicates how well we are managing our inventory levels, with higher inventory turns resulting in more working capital availability and a higher return on our investments in inventory. Our annualized fourth quarter inventory turns for fiscal 2019 decreased to 2.1 turns from 2.2 turns in fiscal 2018 primarily as a result of a decrease in demand for sales of our large ELA tools, partially offset by the impact of lower inventories due to restructuring charges.
Adjusted EBITDA as a Percentage of Net Sales We define adjusted EBITDA as operating income adjusted for depreciation, amortization, stock compensation expense, major restructuring costs and certain other non-operating income and expense items, such as costs related to our acquisitions. Key initiatives to reach our goals for EBITDA improvements include utilization of our Asian manufacturing locations, optimizing our supply chain and continued leveraging of our infrastructure.
We utilize a number of different financial measures, both GAAP and non-GAAP, such as adjusted EBITDA as a percentage of net sales, in analyzing and assessing our overall business performance, for making operating decisions and for forecasting and planning future periods. We consider the use of non-GAAP financial measures helpful in assessing our current financial performance and ongoing operations. While we use non-GAAP financial measures as a tool to enhance our understanding of certain aspects of our financial performance, we do not consider these measures to be a substitute for, or superior to, the information provided by GAAP financial measures. We provide adjusted EBITDA in order to enhance investors’ understanding of our ongoing operations. This measure is used by some investors when assessing our performance.
Table:
 | Fiscal | 
 | 2019 | 2018
Net Sales - OEM Laser Sources | $886,676 | $1,259,477
Net Sales - Industrial Lasers & Systems | $543,964 | $643,096
Gross Profit as a Percentage of Net Sales—OEM Laser Sources | 47.3% | 52.7%
Gross Profit as a Percentage of Net Sales—Industrial Lasers & Systems | 13.3% | 26.7%
Research and Development Expenses as a Percentage of Net Sales | 8.2% | 7.0%
Income From Continuing Operations Before Income Taxes | $60,048 | $361,555
Net Cash Provided by Operating Activities | $181,401 | $236,111
Days Sales Outstanding in Receivables | 67 | 67
Annualized Fourth Quarter Inventory Turns | 2.1 | 2.2
Net Income From Continuing Operations as a Percentage of Net Sales | 3.8% | 13.0%
Adjusted EBITDA as a Percentage of Net Sales | 18.1% | 28.9%


Question: In which years are the key performance indicators provided in the table?
Answer: 2019, 2018