Of the 26 markets for which an independent third party fair value appraisal was obtained in the prior year, one market was sold, leaving 25 markets applicable to the current year. The table below presents the percentage within a range by which our prior year start-up income estimated fair value exceeds the current year carrying value of our broadcasting licenses:

During the third quarter of 2019 we performed an interim review of broadcast licenses in seven markets that reported revenues that were trending below the amounts forecasted in the 2018 valuations. At that time, we also noted station sales indicating potential declines in market-dependent multiples and we believed that downward revenue trends noted during the first half of 2019 were more likely to prevail throughout the remainder of 2019 as compared to earlier quarters when the trends were not as prevalent. Based on our interim review and analysis, we recorded an impairment charge of $1.9 million to the value of broadcast licenses in Louisville, Philadelphia, Portland and San Francisco. The table below presents the percentage within a range by which our September 30, 2019 start-up income estimated fair value exceeds the current year carrying value of our broadcasting licenses:

The second part of our qualitative assessment consists of a review of the financial operating results for each market cluster. Radio stations are often sold on the basis of a multiple of projected cash flow, or Station Operating Income (“SOI”) defined as net broadcast revenue less broadcast operating expenses. See Item 6 – Selected Financial Data within this annual report for information on SOI, a measure. Numerous trade organizations and analysts review these radio station sales to track SOI multiples applicable to each transaction. Based on published reports and analysis of market transactions, we believe industry benchmarks to be in the six to seven times cash flow range. We elected an SOI benchmark of four as a indicator of fair value. Based on this qualitative review, we identified 6 markets subject to further testing that were not tested in the prior year and one market for which a valuation was obtained as part of the broadcast goodwill testing.

Based on our assessment we engaged Bond & Pecaro, an independent third-party appraisal and valuation firm, to assist us with determining the enterprise value of 16 of our market clusters. The estimated fair value of each market cluster was determined using the Greenfield Method, a form of the income approach. The premise of the Greenfield Method is that the value of a broadcast license is equivalent to a hypothetical in which the only asset owned by the station as of the valuation date is the broadcast license. This approach eliminates factors that are unique to our operation of the station, including its format and historical financial performance. The method then assumes the entity has to purchase, build, or rent all of the other assets needed to operate a comparable station to the one in which the broadcast license is being utilized as of the valuation date. Cash flows are estimated and netted against all costs, expenses and investments necessary to achieve a normalized and mature state of operations, thus reflecting only the cash flows directly attributable to the broadcast license. A multi-year discounted cash flow approach is then used to determine the net present value of these cash flows to derive an indication of fair value. For cash flows beyond the projection period, a terminal value is calculated using the Gordon constant growth model and long-term industry growth rate assumptions based on long-term industry growth and Gross Domestic Product (“GDP”) inflation rates.

The table below presents the results of our impairment testing under the income approach at December 31, 2019: