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I. INTRODUCTION Economists and policymakers consider productivity growth to be "the principal source of improvements in living standards." (1) Not surprisingly then, tracking and explaining productivity growth at the national or industry level have a long tradition in economic research. Most studies have focused on explaining differences across countries and over time, with the aim of identifying countries that grow consistently faster than others, or economy-wide factors or circumstances that will help countries enjoy superior productivity growth. Overall, it seems clear that at least some of the variability in aggregate productivity growth is systematic: it is noticeably procyclical and displays some unexpected structural breaks (as, e.g., in the UK productivity growth slowdown in the 1970s). Notwithstanding this variability, however, some countries stand out as being persistently high--productivity growth performers and others as being persistently low, at least over a span of one or more decades (e.g., Crafts and Mills 2005; Hulten 1990), while in some economies phases of "high" and "low" productivity growth can be identified quite clearly (e.g., Nelson and Wright 1992; O'Mahony 1999). Studying aggregate productivity growth may not tell the whole story, however. For example, a country or an industry displaying superior growth could be strong "across the board," with all (or most) firms outperforming their peer group of firms in other countries or industries. In this case, the high average level of productivity growth is driven by the high median level of productivity growth. Alternatively, superior performance could be driven by a small group of above-average performers who outweigh the average performance of the rest. Here, the high average level of productivity growth is well above the median level of productivity growth and is driven by productivity "outliers." What is more, these outliers could be the same firms every year ("productivity champions") of the identity of such superior performers could switch periodically. Such considerations have a significant bearing on growth-enhancing policies and may help guide the search for sources of high productivity growth. If, for example, a small number of firms were consistent strong performers, studying the characteristics of these firms would help uncover some important sources of economy-wide productivity growth. Conversely, if these "bursts" of productivity leadership by a small but unpredictable group of firms were short lived and driven by more of less unpredictable shocks, finding a way to make more firms benefit from these shocks and identifying which types of shocks create a climate conducive to productivity growth could help raise aggregate productivity growth. Taking productivity growth (rather than levels) as the performance variable of interest, it is interesting to ask why certain firms outperform others: that is, what makes a "productivity leader"? However, it is arguably even more interesting to understand what determines how long any particular firm will outperform its peers: that is, what makes a "productivity champion"? Outstanding performance in any one year may well be due to a (combination of a) whole host of exogenous and transitory factors; persistent superior performance is much more likely to reflect permanent, deep-seated sources of competitive ability or advantage. Our paper hopes to make four contributions to the literature. (i) By studying individual firms' productivity growth rather than aggregate growth, we give some insight into the degree of heterogeneity in the performance of firms in an economy--is productivity likely to be driven by a small number of champions or a general climate conducive to high growth? (ii) By measuring superior performance using productivity growth, we avoid the well-known problems of measuring performance through accounting profitability. Accounting profitability is often manipulated by firms to display stable earnings over time, which will exaggerate the stability of corporate performance. …

An Erratum for this article has been published in Journal of Applied Econometrics 18(2) 2003, 249 Previous empirical work on corporate growth rates using cross-section or short-panel econometric techniques suggests that growth rates are random but that some degree of mean reversion exists. This means that size differences between firms are transitory. Another, more natural way to explore the long-run distribution of firm sizes is to examine data on the growth of particular firms over long periods of time. Using a sample of 147 UK firms observed continually for more than 30 years, our conclusions are that growth rates are highly variable over time and that differences in growth rates between firms do not persist for very long. Further, firms show no tendency to converge to either a common size or to a pattern of stable size differences over time. These results are compared and contrasted with standard approaches that suggest that firms reach and maintain stable positions in a skewed size distribution. Copyright © 2002 John Wiley & Sons, Ltd.

Diophantine equations and inequalities are presented for main-group closed-shell diatomic molecules. Specifying various bond types (covalent, dative, ionic, van der Waals) and multiplicities, it becomes possible to identify all possible molecules. While many of the identified species are probably unstable under normal conditions, they are interesting and present a challenge for computational or experimental analysis. Ionized molecules with net charges of -1, 1, and 2 are also identified. The analysis applies to molecules with atoms from periods 2 and 3 but can be generalized by substituting isovalent atoms. When closed-shell neutral diatomics are positioned in the chemical space (with axes enumerating the numbers of valence electrons of the free atoms), it is seen that they lie on a few parallel isoelectronic series.

Due to the repetitive rotational and distractive forces exerted onto the posterior shoulder during the deceleration phase of the overhead throwing motion, limited glenohumeral (GH) range of motion (ROM) is a common trait found among baseball players, making them prone to a wide variety of shoulder injuries. Although utilization of instrument-assisted soft tissue mobilization (IASTM), such as the Graston® Technique, has proven effective for various injuries and disorders, there is currently no empirical data regarding the effectiveness of this treatment on posterior shoulder tightness.To determine the effectiveness of IASTM in improving acute passive GH horizontal adduction and internal rotation ROM in collegiate baseball players.Thirty-five asymptomatic collegiate baseball players were randomly assigned to one of two groups. Seventeen participants received one application of IASTM to the posterior shoulder in between pretest and posttest measurements of passive GH horizontal adduction and internal rotation ROM. The remaining 18 participants did not receive a treatment intervention between tests, serving as the controls. Data were analyzed using separate 2× 2 mixed-model analysis of variance, with treatment group as the between-subjects variable and time as the within-subjects variable.A significant group-by-time interaction was present for GH horizontal adduction ROM with the IASTM group showing greater improvements in ROM (11.1°) compared to the control group (-0.12°) (p0.001). A significant group-by-time interaction was also present for GH internal rotation ROM with the IASTM group having greater improvements (4.8°) compared to the control group (-0.14°) (p0.001).The results of this study indicate that an application of IASTM to the posterior shoulder provides acute improvements in both GH horizontal adduction ROM and internal rotation ROM among baseball players.2b.

Objectives To assess the role of cervical sonography and to compare various sonographic cervical parameters in their ability to predict spontaneous preterm birth in high-risk singleton gestations. Design A prospective cohort of 469 high-risk gestations were longitudinally evaluated between 15 and 24 weeks' gestation on 1265 occasions with transvaginal cervical sonography and transfundal pressure. The cervical parameters obtained were funnel width and length, cervical length, percent funneling and cervical index. The information obtained was used for patient management. Restriction of physical activities was initiated at cervical lengths of ≤ 2.5 cm with cerclage as an option for cervical lengths of ≤ 2.0 cm. Results Receiver operating characteristic curve analyses showed that a cervical length of ≤ 2.5 cm between 15 and 24 weeks' gestation was equal to the other sonographic cervical parameters in its ability to predict spontaneous preterm birth. The sensitivities for delivery at < 28, < 30, < 32 and < 34 weeks' gestation were 94%, 91%, 83% and 76%, respectively, while the negative predictive values were 99%, 99%, 98% and 96%, respectively. The placement of a cerclage did not influence the positive and negative predictive values. In comparison to women with other risk factors, cervical length was best in the prediction of preterm birth in women with a prior mid-trimester loss; an optimal cut-off of ≤ 1.5 cm had sensitivities for delivery at < 28, < 30, < 32 and < 34 weeks' gestation of 100%, 100% 92% and 81%, respectively. The rate of preterm delivery at < 34 weeks' gestation increased dramatically when the cervical length was ≤ 1.5 cm. Cervical length was the only independent variable that entered the logistic regression model for the prediction of preterm delivery at < 34 weeks' gestation. Conclusions In high-risk singleton gestations a cervical length of ≤ 2.5 cm was equal to other sonographic cervical parameters in its ability to predict spontaneous preterm birth and was better for the prediction of earlier forms of prematurity (at < 28 and < 30 weeks) than later forms (at < 32 and < 34 weeks). The optimal cervical lengths and their performance for predicting prematurity may be influenced by obstetric risk factors. Copyright © 2001 International Society of Ultrasound in Obstetrics and Gynecology

Objective To compare pregnancy outcomes in women at risk for pregnancy loss treated with elective versus ultrasound-indicated placement of cerclage. Methods A retrospective cohort study was performed on two groups of patients with singleton gestations. The first group consisted of women at risk for pregnancy loss who were treated with an elective cerclage, while the second group was managed conservatively and followed with serial transvaginal cervical sonography and transfundal pressure. An emergency cerclage was placed in women in the second group when the endocervical canal length shortened to < 20 mm, either spontaneously or in response to transfundal pressure. The two groups were compared with respect to maternal demographics, obstetric and gynecological history, and gestational age, both at time of cerclage placement and delivery. Results A total of 138 patients were identified. Eighty-one patients were treated with an elective cerclage and 57 with an ultrasound-indicated cerclage. Patients treated with elective cerclages were older (32 versus 27 years, p = 0.0003), more commonly white (56.8% versus 38.6%, p = 0.0380), less commonly nulliparous (23.5% versus 43.9%, p = 0.0063), and more often private patients (92.6% versus 28.1%, p < 0.0001). A history of previous treatment with cerclage (45.7% versus 10.5%, odds ratio (OR) 0.2, 95% confidence interval (CI) 0.1–0.4) and one prior midtrimester loss (53.1% versus 33.3%, OR 0.4, 95% CI 0.2–0.9) were also more common in the elective versus ultrasound-indicated cerclage group. However, there was no difference in the rates of previous preterm delivery, two midtrimester losses, two terminations of pregnancy, in utero diethylstilbestrol exposure, uterine anomalies, history of cone biopsy or parity. As expected, gestational age at placement of cerclage was significantly earlier in the elective group (13.0 versus 20.0 weeks, p < 0.0001). The median (range) gestational age at delivery (37 (15–41) versus 37 (17–41) weeks, p = 0.90), the number of early (< 25 weeks) losses (9.9% versus 8.8%, OR 1.6, 95% CI 0.3–7.9), and preterm deliveries (< 37 weeks) (35.8% versus 36.8%, OR 1.1, 95% CI 0.4–3.2) were similar in the elective and ultrasound-indicated cerclage patients, respectively. Conclusion In patients at risk for pregnancy loss, placement of cervical cerclages in response to ultrasono-graphically detected shortening of the endocervical canal length is a medically acceptable alternative to the use of elective cerclage. Copyright © 1998 International Society of Ultrasound in Obstetrics and Gynecology

This paper considers the relative productivity growth performance of a balanced sample of 80 large UK firms over 9 years (1986-95), using two different benchmarks from which to compute productivity growth differences. We find that there is almost no persistence in superior productivity growth performance however measured - firms that enjoy high productivity growth rates relative to (say) the average in one year, are as likely as not to display sub-average performance in the following year. Our examination of the determinants of the length of time in which particular firms outperform the average suggests that innovative firms carrying low debt who are relatively free from financial distress are likely to display whatever persistently superior performance we observe in the data.

The identification of main-group molecules having atoms with closed valence p shells, i.e., having atoms with eight valence electrons, is continued into the realm of four-atom molecules. All possible covalently-bonded species, obtained from two independent computer programs, are shown. The method for generalizing to molecules in which some or all atoms have closed valence s shells, i.e., having atoms with two valence electrons, is recalled. A list of all prototype linear/bent four-atom molecules, with dative bonds in addition to covalent and/or van der Waals bonds, is presented (permutations having been culled out). A program code, lists of molecules based on other graphs, and lists of five- and covalently-bonded six-atom species, are available on the Web. For molecules derived from the two other four-vertex graphs, their vast numbers require extensive indexing schemes for the results to be useful. The paper concludes with some preliminary observations concerning the stabilities of four-atom molecules having atoms with closed shells.

Studies of the innovative activity of firms which focus on RD that is, finding independent variables that display a high ratio of within to between variation, plus a tendency towards irregular bursts of sub- or supernormal activity. For economists interested in the determinants of innovative activity, this is likely to rule out factors such as ‘technological opportunity’, ‘conditions of appropriability’ or market structure (all of which tend to be different for different firms or industries, but stable over time). However, potential determinants of innovation, such as demand or the financial state of firms, are both interesting on theoretical grounds and potential candidates on purely statistical grounds.

This paper examines the innovative history of a number of UK firms using two large databases, looking for evidence consistent with the view that firms that innovate typically do so persistently. The first sample contains 3304 firms which registered at least one patent in the US at any time in the period 1969–1988, while the second consists of 1624 firms which produced at least one major innovation at any time in the UK from 1945 to 1982. Both data sets yield the same conclusion, namely that very few innovative firms are persistently innovative.

Objective To compare pregnancy outcomes in women at risk for pregnancy loss treated with elective versus ultrasound-indicated placement of cerclage. Methods A retrospective cohort study was performed on two groups of patients with singleton gestations. The first group consisted of women at risk for pregnancy loss who were treated with an elective cerclage, while the second group was managed conservatively and followed with serial transvaginal cervical sonography and transfundal pressure. An emergency cerclage was placed in women in the second group when the endocervical canal length shortened to < 20 mm, either spontaneously or in response to transfundal pressure. The two groups were compared with respect to maternal demographics, obstetric and gynecological history, and gestational age, both at time of cerclage placement and delivery. Results A total of 138 patients were identified. Eighty-one patients were treated with an elective cerclage and 57 with an ultrasound-indicated cerclage. Patients treated with elective cerclages were older (32 versus 27 years, p = 0.0003), more commonly white (56.8% versus 38.6%, p = 0.0380), less commonly nulliparous (23.5% versus 43.9%, p = 0.0063), and more often private patients (92.6% versus 28.1%, p < 0.0001). A history of previous treatment with cerclage (45.7% versus 10.5%, odds ratio (OR) 0.2, 95% confidence interval (CI) 0.1–0.4) and one prior midtrimester loss (53.1% versus 33.3%, OR 0.4, 95% CI 0.2–0.9) were also more common in the elective versus ultrasound-indicated cerclage group. However, there was no difference in the rates of previous preterm delivery, two midtrimester losses, two terminations of pregnancy, in utero diethylstilbestrol exposure, uterine anomalies, history of cone biopsy or parity. As expected, gestational age at placement of cerclage was significantly earlier in the elective group (13.0 versus 20.0 weeks, p < 0.0001). The median (range) gestational age at delivery (37 (15–41) versus 37 (17–41) weeks, p = 0.90), the number of early (< 25 weeks) losses (9.9% versus 8.8%, OR 1.6, 95% CI 0.3–7.9), and preterm deliveries (< 37 weeks) (35.8% versus 36.8%, OR 1.1, 95% CI 0.4–3.2) were similar in the elective and ultrasound-indicated cerclage patients, respectively. Conclusion In patients at risk for pregnancy loss, placement of cervical cerclages in response to ultrasono-graphically detected shortening of the endocervical canal length is a medically acceptable alternative to the use of elective cerclage. Copyright © 1998 International Society of Ultrasound in Obstetrics and Gynecology