As you\\\'ll have gathered, French employment practices contain some of the most byzantine concepts and language around. First, a few definitions relating to \\\"effectif\\\":

\\\"effectif\\\" = \\\"manpower, workforce, staff\\\"
\\\"faire parties des effectifs\\\" = \\\"to be on the payroll\\\"
\\\"maintenir le liveau des effectifs\\\" = \\\"maintain staffing levels\\\"

…and a suggested rendering: \\\"As required by law, salaried employees and wage earners are \\\"included on the payroll.\\\" In order to take into account the characteristic features of such surveys, this inclusion is calculated on the basis of a ratio between the gross earnings of all employees and their traditional theoretical gross earnings.\\\"

A few alternatives:
\\\"included among the workforce\\\" for \\\"included on the payroll\\\"
\\\"take into consideration\\\" or \\\"reflect\\\" for \\\"take into account\\\"
\\\"specific characteristics\\\" for \\\"characteristic features\\\"

Cheers and bonne chance! -- HC