import pandas as pd


def spend_mean(spend: pd.Series) -> float:
    """Shows function creating a scalar. In this case it computes the mean of the entire column."""
    return spend.mean()


def spend_zero_mean(spend: pd.Series, spend_mean: float) -> pd.Series:
    """Shows function that takes a scalar. In this case to zero mean spend."""
    return spend - spend_mean


def spend_std_dev(spend: pd.Series) -> float:
    """Function that computes the standard deviation of the spend column."""
    return spend.std()


def spend_zero_mean_unit_variance(spend_zero_mean: pd.Series, spend_std_dev: float) -> pd.Series:
    """Function showing one way to make spend have zero mean and unit variance."""
    return spend_zero_mean / spend_std_dev