def select_sample_df(list_of_groups:list, 
                    df):
    """Selects specific groups from df
    Args:
    list_of_groups: group numbers in a list, if just one group, specify as a list anyway
    df: pandas DataFrame
    
    Returns: small dataframe with the selected groups and reindexed
    """
    small_df = df[df["group_id"].isin(list_of_groups)].reset_index(drop=True)
    return small_df