def _classification_column_names(otu_defs):
    """
    column names for a csv file being used for a classification problem
    between two cohorts.
    First column is 'cohort' (predicted variable).
    Remaining columns are otu names (input features)
    """
    column_names = list()
    column_names.append('cohort')
    for otu_def in otu_defs:
        otu_name = otu_def.get_value('tornado_observation_key')
        column_names.append(otu_name)
    return column_names