Table 3.

Main performance indicators about the 1990–2021 collection

Main information about data	1990s	2000s	2010s	2020s	Whole period
Timespan	1990–1999	2000–2009	2010–2019	2020–2021	1990–2021
Sources	116	383	829	420	1146
Documents	317	1972	6995	1847	11,131
Average years from publication	24.9	16.2	6.91	1.5	8.17
Average citations per documents	97.1	56.03	29.64	7.035	32.48
Average citations per year per doc	3.703	3.238	3.827	2.65	3.524
References	5452	42,321	216,569	77,960	326,606
Document contents
Keywords plus (ID)	1177	10,567	27,361	10,714	35,560
Author’s keywords (DE)	380	3138	10,562	4359	15,102
Authors
Authors	516	3780	12,677	5054	18,116
Author appearances	1121	8144	36,030	10,212	55,507
Authors of single-authored documents	20	37	53	13	111
Authors of multi-authored documents	496	3743	12,624	5041	18.005
Authors’ collaboration
Single-authored documents	31	53	68	13	165
Documents per author	0.614	0.522	0.552	0.365	0.614
Authors per document	1.63	1.92	1.81	2.74	1.63
Co-authors per documents	3.54	4.13	5.15	5.53	4.99
Collaboration index	1.73	1.95	1.82	2.75	1.64