def standardize_subject_id(sub_id):
    """Standardize subject ID to start with the prefix 'sub-'.

    Parameters
    ----------
    sub_id : str
        subject ID.

    Returns
    -------
    str
        Standardized subject IDs.
    """
    return sub_id if str(sub_id).startswith("sub-") else "sub-" + str(sub_id)