Database schema:

CREATE TABLE "Author" (
	"Id" INTEGER -- example: ['9', '14'],
	"Name" TEXT -- example: ['Ernest Jordan', 'K. MORIBE'],
	"Affiliation" TEXT -- example: ['Department of Molecular Biology|School of Science|', 'Cavendish Laboratory|Cambridge University'],
	PRIMARY KEY ("Id")
)


CREATE TABLE "Conference" (
	"Id" INTEGER -- example: ['2', '1'],
	"ShortName" TEXT -- example: ['IADIS', 'IADT'],
	"FullName" TEXT -- example: ['International Association for Development of the I', 'Issues and Applications of Database Technology'],
	"HomePage" TEXT -- example: ['', 'http://www.informatik.uni-trier.de/~ley/db/conf/ia'],
	PRIMARY KEY ("Id")
)


CREATE TABLE "Journal" (
	"Id" INTEGER -- example: ['2', '1'],
	"ShortName" TEXT -- example: ['ICOM', 'AEPIA'],
	"FullName" TEXT -- example: ['Inteligencia Artificial,revista Iberoamericana De ', 'Zeitschrift Für Interaktive Und Kooperative Medien'],
	"HomePage" TEXT -- example: ['http://aepia.dsic.upv.es/revista/', 'http://www.i-com-media.de'],
	PRIMARY KEY ("Id")
)


CREATE TABLE "Paper" (
	"Id" INTEGER -- example: ['2', '1'],
	"Title" TEXT -- example: ['A nonlocal convection–diffusion equation', 'Stitching videos streamed by mobile phones in real'],
	"Year" INTEGER -- example: ['2007', '2009'],
	"ConferenceId" INTEGER -- example: ['0', '167'],
	"JournalId" INTEGER -- example: ['0', '7234'],
	"Keyword" TEXT -- example: ['mobile video capturing|real-time|video stitching', 'Nonlocal diffusion; Convection–diffusion; Asymptot'],
	PRIMARY KEY ("Id"), 
	CONSTRAINT fk_paper_conference FOREIGN KEY("ConferenceId") REFERENCES "Conference" ("Id"),
	CONSTRAINT fk_paper_journal FOREIGN KEY("JournalId") REFERENCES "Journal" ("Id")
)


CREATE TABLE "PaperAuthor" (
	"PaperId" INTEGER -- example: ['5', '4'],
	"AuthorId" INTEGER -- example: ['1102257', '1456512'],
	"Name" TEXT -- example: ['Kazuo Iwama', 'ADAM G. JONES'],
	"Affiliation" TEXT -- example: ['The french National Institute for Research in Comp', 'Kyushu University 812 Fukuoka Japan 812 Fukuoka Ja'],
	CONSTRAINT fk_paperauthor_paper FOREIGN KEY("PaperId") REFERENCES "Paper" ("Id"),
	CONSTRAINT fk_paperauthor_author FOREIGN KEY("AuthorId") REFERENCES "Author" ("Id")
)

Question: Calculate the average of authors for each paper from the year of 1990 to 2000.
Evidence: Average refer to DIVIDE(COUNT(AuthorID where Year = 1990 BETWEEN 2000), COUNT(Title where Year = 1990 BETWEEN 2000))

SQL:
SELECT CAST(COUNT(DISTINCT T2.AuthorId) AS REAL) / COUNT(DISTINCT T1.Title) FROM Paper AS T1 INNER JOIN PaperAuthor AS T2 ON T1.Id = T2.PaperId WHERE T1.Year BETWEEN 1990 AND 2000