java, calculate standard deviation?

here's my method:

private double calculateStandardDeviation(){

double mean = calculateMean();

double totalOfSquaredDifferences = 0;

for(Employee employee: employeeList){

totalOfSquaredDifferences += Math.pow(mean - (employee.getHoursWorked() * employee.getRate()), 2);

}

double variance = totalOfSquaredDifferences / employeeList.size();

return Math.sqrt(variance);

}

but im getting the wrong answer and can't spot a logic error

hep!

thanks!