1.3 Citation Impact
In assessing a state’s research performance, it is important to take into account both the volume and the quality of research output. Citations are widely recognized as a proxy for quality.
A publication usually cites or makes formal references to previous works upon whose findings or ideas the research builds. The number of citations a publication receives from subsequently published articles is often interpreted as a proxy of the quality or importance of that publication.
Since it takes time for publications to accumulate citations, it is normal that the total number of citations for a state’s cumulative publications is lower for the most recent years. Moreover, different states have different research strengths, and citations in research from one field may accumulate faster than others because that field simply produces more publications. Therefore, instead of comparing absolute counts of citations across years and states, this report recommends using a citation measure called field-weighted citation impact (also known as FWCI) that adjusts for these differences.
Field-weighted citation impact divides the number of citations received by a publication by the average number of citations received by publications in the same field, of the same type, and published in the same year.10 The world average is indexed to a value of 1.00. Values above 1.00 indicate above-average citation impact, and values below 1.00 likewise indicate below-average citation impact. For example, a state with a field-weighted citation impact of 1.16 indicates that the average paper from that state was cited 16 percent above the world average whereas a state with a field-weighted citation impact of 0.91 indicates that the average paper from that state was cited 9 percent below the world average.
The overall field-weighted citation impact of all U.S. research output from 2004 to 2013 was 1.49. Figure 1.6 shows a map of all of the states and their respective field-weighted citation impacts. Massachusetts and Washington achieved the highest field-weighted citation impacts among all states at 2.10 and 2.03, respectively. Other states with high field-weighted citation impacts for their respective regions include California (1.94, third overall and second among all states in the West), Maryland (1.90, fourth overall and second among all states in the East), Minnesota (1.86, fifth overall and first among all states in the Midwest), and North Carolina (1.80, seventh overall and first among all states in the South). In contrast to Figure 1.1, there is a much more even distribution of highly impactful research throughout the country.
While the relative positions of states on this measure are mostly stable over time, some states significantly improved the citation impact of their research over the past ten years. For example, the field-weighted citation impact of Tennessee’s research output grew 1.50 percent per year from 1.54 in 2004 (25th among all states) to 1.76 in 2013 (14th among all states). This was the highest growth rate in field-weighted citation impact among all states that had a field-weighted citation impact above the U.S. average.
More importantly, as the next section illustrates, different states—and not necessarily those with the highest research expenditures or outputs—have comparative advantages in different fields.
Although there is a positive correlation between a state’s research output and its field-weighted citation impact (see Figure 1.7), many states have field-weighted citation impacts that are higher than one would otherwise predict from a linear regression. This includes both those with smaller absolute levels of output such as Rhode Island, New Hampshire and Vermont and those with larger absolute levels such as Minnesota and Washington.
Figure 1.6—Field-Weighted Citation Impact for U.S. States, 2004–2013. Source: Scopus®
Figure 1.7—Publication Output Versus Field-Weighted Citation Impact for U.S. States, 2004–2013. Source: Scopus® Normalized from 0 to 1 by percentile. Best-fit straight line added.
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