Cleaning Data

Cleaning Data

The data that you get is rarely as accurate and as correct as you'd like.

Cleaning Data

Taking the data you find and removing/​fixing problem data is called data cleaning (or data cleansing). It is often a big part of a real-world data analysis task (maybe 50–80%).

The process is generally fairly ad hoc: what you need to do depends on the data you get.

Cleaning Data

Example problems: Data Cleaning & Integration, Chau

• duplicates
• empty rows
• (different) abbreviations
• difference in scales/​inconsistency in description/​sometimes include units
• typos
• missing values
• trailing spaces
• incomplete cells
• synonyms of the same thing
• skewed distribution, outliers
• bad formatting/​not in relational format

Cleaning Data

In general, it's all just programming.

There are a few common problems and solutions we can talk about…

Validity

First problem: is the data valid?

• Actually present?
• Correct data type? number, boolean, etc.
• A legal value? length = -10? Date = February 31 2019? Phone number = 6 or 555-5555?

If you have invalid data in your data set, you can fix, delete, etc, as appropriate.

Outliers

An outlier is a data point that's far away from the others.

Exactly how you define far away is going to depend on the data. What you do with outliers is also going to depend…

Outliers

Outliers can occur in perfectly valid data that has no problems.

data = np.random.normal(0, 1, 1000)
print(data.mean(), data.std())
print(data.min(), data.max())
plt.boxplot(data, notch=True, vert=False)
-0.0478499351952 1.00437194396
-2.65944945638 4.10869262381

Outliers

The point at $$x\approx 4$$ is four standard deviations from the mean. That would certainly be be considered an outlier.

But we shouldn't just ignore it: it's a perfectly valid sample and should be included in any analysis.

Outliers

Important point: you can't remove a data point just because you don't like it!

It makes sense to remove data if it's genuinely an error in measurement, or isn't fundamentally a true measurement of what you're looking at.

If it's just a slightly weird value, you have to justify it as an outlier, or keep it.

Outliers

e.g. Suppose I am examining the temperature in my house, and take some measurements:

ObjectTemperature
Air, bedroom21°C
Air, living room19°C
Air, oven120°C
Desk surface25°C

One value is clearly an outlier that measures a different thing. One is strangely high, but probably a valid measurement.

Finding Outliers

You can identify outliers by looking at how far a point is from the mean, often how many standard deviations from the mean.

But that probably won't tell the whole story: different types of problem will show up in different ways.

Finding Outliers

Suggestion: start by plotting.

Get a feeling for how your data is shaped, and look for problems by eye.

Finding Outliers

My scatter plot of date vs dog ratings for Exercise 2, without removing outliers:

Finding Outliers

After removing the crazy outliers, we can actually see the data, and it looks okay:

Finding Outliers

Or a box plot, with and without outliers:

Or a histogram:

Finding Outliers

With a basic plot, it seems pretty obvious that some of the “$$n/10$$” ratings aren't really valid.

Including them in the analysis would overwhelm any other trends. We removed them from the data set.

Handling Outliers

A few things you can do if you identify outliers:

• Leave it as-is because it's actually valid.
• Remove the record entirely because it's invalid.
• Remove the value at treat it as a missing value (Pandas NaN or similar) in that record.
• Impute…

Imputation

If you have missing values (or deleted outliers), you could replace them by calculating a plausible value to replace them. This is called imputation (imputing, to impute).

Imputation

Imputing is probably a bad idea: it's possible to introduce bias, introduce fake trends, wash out real trends, ….

Sometimes it's the least-bad option available.

Imputation

Common ways to impute:

• Use (or average) nearby values.
• Average of known data. (mean substitution)
• Linear regression of nearby values. *
• Some other best-fit or maximum-liklihood result.

* i.e. do LOESS smoothing, and take the filtered prediction at that point.

Imputation

Imputing can be useful when you have many variables and don't want to throw away a record for a relatively-low-importance variable.

Imputation

It might also be relevant to leave missing values in your data set. Maybe the fact that a value was missing is itself an important fact, and could be useful in later analysis?

Noise Filtering

As we talked about in the Noise Filtering notes.

Entity Resolution

Entity resolution or record linkage is the process of finding multiple values that actually refer to the same concept (entity).

e.g. Are Greg Baker and Gregory G Baker the same person?

Entity Resolution

e.g. Are these the same product? How could I (automatically) compare prices?

Entity Resolution

In general, entity resolution is hard. Hope you have some structure or more precise values to work with.

e.g. employee/​student number.

e.g. model number YD170XBCAEWOF. But are item model number and model always the same concept?

Entity Resolution

An approximate string matching algorithm might help, like difflib.get_close_matches or FuzzyWuzzy.

Or maybe you know about your data. e.g. two stores with the same name less than 500 m apart are unlikely: they're probably the same store.

Regular Expressions

Often, what you get is strings that have something useful in them. e.g. tweets, scraped HTML pages.

If there's a little structure, a regular expression can be a good way to extract what you need.

Regular Expressions

For example, looking for tweets with text vote for party or voting for party. This can be done with Series.str.extract:

tweets = pd.DataFrame({'text': [
"I'm voting for Green",
'Get out and vote for @terrybeech!',
'How do I vote?'
]})
parties = tweets['text'].str.extract(
r'vot(e|ing) for (.*)', expand=False)[1]
print(parties)
0           Green
1    @terrybeech!
2             NaN
Name: 1, dtype: object

Regular Expressions

… or with Python's re:

import re
party_re = re.compile(r'vot(e|ing) for (.*)')
def get_party(txt):
match = party_re.search(txt)
if match:
return match.group(2)
else:
return None

parties = tweets['text'].apply(get_party)
print(parties)
0           Green
1    @terrybeech!
2            None
Name: text, dtype: object

Python re

A few basics about Python's regular expression library (which is the same syntax as Pandas' regular expressions)…

Regular expressions contain a lot of backslashes, but those are special in Python strings. It's common to use Python raw string literals for regex patterns

'\n' # one character: newline
r'\n' # two characters: backslash, n

Python re

If you're using a regex a lot (and you probably are), you should compile it once and use many times:

import re
input_pattern = re.compile(r'…')

Then you can use the .match() (look at start of string) or .search() method (look anywhere in the string) to find the pattern (or not):

m = input_pattern.match(input_string)
if m:
# handle the match
else:
# no match

Python re

Some things you can do in regular expressions:

• r'x' matches an x character
• r'.' matches any single character
• r'\.' matches a single . character
• r'x*' matches 0 or more x
• r'x+' matches 1 or more x
• r'x?' matches x but it's optional (0 or 1)

Python re

Some things you can do in regular expressions:

• r'^' matches start of string

Python re

But could there be IPv6 addresses? Is HTTP/\d\.\d always going to match the HTTP version? Other optional stuff?

Suggestion: be a little defensive.

with open('re-input.txt') as logfile:
for line in logfile:
m = log_pattern.match(line)
if not m:
raise ValueError('Bad input line: %r' % (line,))
# ⋮

Python re

But we're going to actually want to extract the parts. A (…) pattern captures the … part so it can be extracted later.

log_pattern = re.compile(
r'^(\d+\.\d+\.\d+.\d+) "GET (\S+) HTTP/\d\.\d" (\d+)$') Then on a match object, m.group(1) will be the IP address match, etc. If using Series.str.extract, it will become column '0' in the result. Python re log_pattern = re.compile( r'^(\d+\.\d+\.\d+.\d+) "GET (\S+) HTTP/\d\.\d" (\d+)$')
with open('re-input.txt') as logfile:
for line in logfile:
m = log_pattern.match(line)
if not m:
raise ValueError('Bad input line: %r' % (line,))
print((m.group(1), m.group(2), m.group(3)))
('192.168.10.11', '/', '200')
('10.11.12.13', '/missing.html', '404')
('192.168.10.11', '/page.html', '200')

Regex Summary

You can get a lot done with regular expressions.

… but don't get fooled into thinking they're the best tool for every string processing task.