def filter_tickers(df, na_rate=0.1, thresh=20):
    """
    filter tickers by applying threshold to na values of price
    """
    chosen_columns = df.columns[df.isna().mean() < na_rate]
    chosen2 = chosen_columns[~df[chosen_columns].pct_change().eq(0).rolling
        (thresh).sum().eq(thresh).any()]
    return df[chosen2].ffill()
