本研究以1986年至2014年美國上市製造業廠商為研究對象，探討外部融資和研發的相關性。由於技術進步和創新是經濟成長的重要動力，除了提高產業的附加價值，也能提升國家競爭力，對經濟發展極為重要(Akcali and Sismanoglu, 2015)。然而研發投資相對於其他固定資產投資，是屬於不確性較高的投資項目，且無法當作抵押品，相對可能更容易受到金融摩擦的影響(Hsieh and Huang,2020)，因此我們推測廠商的外部融資應是決定研發投資行為的因素之一。
由於美國的資本市場較先進，有利於廠商本身的投資活動，但對於研發投資而言，可能會因行業不同而有差異。Hsu, Tian and Xu, (2014)指出，專利能為製造業廠商帶來更高價值，因此他們會更傾向透過申請專利保護研發成果。所以本研究選擇以美國上市製造業廠商作為研究對象，並探討其外部融資和研發的相關性。
除了以研發支出作為被解釋變數外，我們亦使用專利數據進行分析。Arora, Belenzon and Sheer (2021) 修正了NBER專利數據庫中，因公司合併而造成專利歷史數據遺失的問題，並合併Compustat資料庫中的公司代碼與CRSP資料庫的專利受讓人編號，使學者能結合公司財務面與專利數據進行分析。由於專利申請的流程較有統一的標準，且具有完整的資料庫，使我們能更精確地衡量廠商每年的專利資料，因此本研究使用專利數據作為衡量研發產出的指標。
上述提及，由於研發是屬於不確定性較高的投資項目，我們推測，若外部融資和研發投資的相關性高，當金融危機發生時，廠商很可能會因融資機會減少，進而影響研發投資。因此本研究亦加入經濟政策不確定指標(Economic Policy Uncertainty)作為市場不確定的變數。實證結果顯示，平均而言，美國上市製造業廠商的外部融資和研發投資為正相關；不確定性和廠商的研發投資也具有正向相關性。然而當金融危機發生時，意味著不確定性指數增加到某個水平，廠商可能會受到信貸市場緊縮的影響，導致借貸不易，進而減少研發投資活動。

This paper analyzes the relationship between external financing and innovation, using data on manufacturing listed firms in the United States from 1986 to 2014. Technological progress and R&D investment are essential catalyst of economic growth ( Akcali and Sismanoglu, 2015). However, the return of R&D investment is highly uncertain and, the investment is lack of collateral value. It will likely to make R&D investment more susceptible to financing frictions (Hsieh and Huang,2020). Therefore, external finance may be an important factor that impacts firm’s innovation investment activities.
Capital markets in the United States are well developed. However. R&D activities may be different across industries. Comparing to the other industries, patents are most valuable and crucial to manufacturing industries. (Hsu,Tian and Xu,2014). We evaluates relationship between the R&D expense and firm’s external finance.
In addition, we employ the patent data from NBER patent database. Arora, Belenzon and Sheer (2021) improve the database and link each PERMNO to Compustat GVKEY code. It is valuable for all researchers working with patent data linked to firms. Since patent statistics is rooted in their wide availability. It's appropriate to use the firms patent data as a proxy for innovative output.
As mentioned above, innovation investment is highly uncertain. It implies that , the firms are likely more difficult to obtain R&D funding if a financial crisis occurs. Therefore we evaluate the correlation between external financing and innovation with the uncertainty index. Our empirical results imply that innovation investment are positively correlated with external financing and uncertainty index. However, the firms may be difficult to obtain R&D funding when the financial crisis occurs, thereby reducing R&D investment activities.

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