Research1 has shown that almost all the variation in the return of an investment portfolio can be explained by the portfolio's exposure to three factors:
Shares have higher expected returns than fixed income
Shares in small companies have higher expected returns than shares in large companies
Shares in lower priced 'value' companies have higher expected returns than shares in higher priced 'growth' companies
Jessop uses this research to create highly diversified portfolios with exposure to as many as 10,000 different securities
1 Dimensional Fund Advisors study 2002