When you are citing a statistic or other specific fact, a signal phrase is often not necessary. In most cases, readers will understand that the citation refers to the statistic or fact (not the whole paragraph).
According to a 2002 survey, 60% of responding companies reported disciplining employees who had used the Internet in ways the companies deemed inappropriate; 30% had fired their employees for those transgressions (Greenfield and Davis 347).
There is nothing wrong, however, with using a signal phrase to introduce a statistic or other fact.
Exercise: Integrating sources in MLA papers 1
Exercise: Integrating sources in MLA papers 2
Exercise: Integrating sources in MLA papers 3
Exercise: Integrating sources in MLA papers 4