The h-index and N the total citable items were calculated for the 161 AG, 60 CMP, 124 GH, and 38 MP journals. The h-index and N show a linear relationship in a double logarithm plot (Figure ), that is, the more a journal publishes the more likely it is to have highly cited works and a high h-index. The slope of this relationship was 0.57 and the R2 was 55%. Journals from the four different fields cannot be separated into clusters in this plot. Journals with a high h-index for their size represent those with a higher than average track record for publishing articles that are well cited.
A double logarithm plot of the h-index and the number of citable items in a journal since 1986. The line of best fit has a slope of 0.57.
To illustrate the relationship between h-index, IF and N, the maximum h-indices for a GH and an AG journal were 295 and 100 respectively, with many AG journals having a higher h-index than GH journals – only 17 of the 124 GH journals had h ≥ 100. Moreover, when an AG journal was matched to its nearest GH journal for both h and N, the GH journal had a higher impact factor. For example, the pair of the Australian Journal of Agricultural Research and Animal Genetics had almost identical h and N (44 & 2549 compared to 44 & 2429) but the IF were 0.993 and 2.437 respectively.
To determine if there were any features of the SRI that might be common to all fields, the SRI for journals from all fields were ranked on one scale from 1 to 383 and plotted in ascending order (Figure ). When all the journals are plotted on the one axis (Figure combined) it is obvious that the graph has flat shoulders, or a bend at SRI ~ 4 and at SRI ~ 6, with most journals having an SRI between 3.0 and 7.0. Because this is a graph of ranks, it means that there are relatively few journals with SRI < 4 or SRI > 6. To determine whether the form of the graph was the same in all 4 fields, the SRI of each journal was separated into its field but was still plotted against its overall rank across all fields (Figures Agriculture – Genetics). This allows the relative position of each journal to be seen clearly both within and across fields, as well as any consistent differences between fields. In all fields, most journals occurred between SRI ~ 4 and SRI ~ 6. A similar shape of the graph is seen in each field, although smaller fields show sparser plots. GH seems to have slightly more journals with SRI between 5 and 6, as can be seen in the density of the Genetics graph compared to the others. In contrast to the similarity between maximum and minimum SRI values in the different fields, the maximum IF in AG, CMP, GH and MP was 3.063, 17.857, 25.797 and 3.584 respectively.
The ranking of the strike rate index across four fields.
The four fields showed similar median values for SRI but not similar medians for IF. Journals for AG, CMP, GH and MP showed medians of 4.4, 4.6, 5.1 and 4.5 respectively. This compares to median impact factors for these fields of 0.57, 0.97, 2.68 and 1.10 [8
], a five fold range. There were more AG journals below 4, although the highest percentage was in MP (34%), and more GH journals above 6, although the two highest journals were review journals in CMP.
Most of the journals with SRI ≥ 6.0 were review journals (14 out of 20), most of which were in GH (n = 8). Most of the journals with SRI ≤ 4.0 were regional or published infrequently. Agriculture has, understandably, more regional journals than the other fields in both absolute numbers (41/161) and in percent, and has the largest number but not percent of journals with SRI below 4 – one should not generalise here since some of the best AG journals also have a country name in their title.
The SRI is not particularly biased against young journals in fast growing fields. Genome Biology, an Open, web based journal, is a case in point, 2005 is the first year it received an IF (9.712) and it has an SRI already of 5.34. While this is not yet up to that of Genome Research (IF 10.139, SRI 6.05), the journal is now in its 8th volume. BMC Genomics is the highest of the other Open GH journals, with IF 4.092 and SRI 4.75, which compares to Genomics with IF 3.181 and SRI 5.45. While BMC Genomics has the higher impact factor it does not yet have the track record of Genomics of publishing highly cited articles.
Once the SRI is above approximately 4.5, the IF shows little relationship to the SRI (Figure ) or the record of the journal for publishing highly cited work. Journals with very similar SRI will show markedly different IF. Journals in GH or CMP with very high IF did not show correspondingly higher SRI either to other journals in GH or CMP respectively, or to journals in AG or MP. Apart from those journals with high IF in these two fields, the plot of SRI by IF shows no differentiation between fields.
The relationship between strike rate index and impact factor across four fields.
SRI explained a greater percentage of the variance in CC in cattle QTL studies than IF and, unlike IF, showed a significant slope of increasing CC with increasing SRI. There were a total of 375 citations to cattle QTL studies for 58 articles published in 2003 and 2004. The plot of CC versus IF is shown in Figure . The slope of the robust regression of CC on IF was 0.34 (s.e. 0.30, t = 1.13 n.s.), and the model explained a non-significant 1.7% of the variance. Of these articles, 37 had between 0 and 5 citations and of these, eight had 0 or 1 citations. When grouped into journal type, the mean CC were significantly different. For citations for cattle QTL work published in 2003 and 2004, the mean CC for AG was 7.67 (N = 27, s.e.m. = 1.42), for GH was 4.79 (N = 28, s.e.m. = 0.99) and for MD was 11.33 (N = 3, s.e.m. = 4.26). These differences are significant with F = 2.3, P < 0.05. The CC for cattle QTL papers showed a strong trend when plotted against SRI (Figure ). The slope of the robust regression of CC on SRI was 1.09 (s.e. 0.12, t = 8.80 P < 0.001) and the model explained a significant 6.9% of the variance (P < 0.05). The SRI explained at least 4 times as much of the variation in CC compared to the nominal variation explained by IF.
The relationship of citations for cattle QTL articles published in 2003–4 against (a) the impact factor and (b) the strike rate index of the journals in which they were published.