def getOpString(mean, std_dev):

    """
    Generate the Operand String to be used in workflow nodes to supply 
    mean and std deviation to alff workflow nodes

    Parameters
    ----------

    mean: string
        mean value in string format
    
    std_dev : string
        std deviation value in string format


    Returns
    -------

    op_string : string


    """

    str1 = "-sub %f -div %f" % (float(mean), float(std_dev))

    op_string = str1 + " -mas %s"

    return op_string