def normalize_z(df, feature):
    """Takes DataFrame with measurements and feature name and adds a column with normalized values of the feature.
    """
    mean = df[df.Status == 'Sample'][[feature]].mean()
    std = df[df.Status == 'Sample'][[feature]].std()
    df[feature + '_norm'] =  df[feature].apply(lambda x:(x - mean)/std)
    return(df)