Addresses written many ways

The same place is written with different abbreviations, word order, capitalization, and levels of detail.

What it looks like

Before

street	unit	city	region	postal_code
123 Main St.	·	Springfield	IL	62701
123 MAIN STREET	·	springfield	Illinois	62701
123 Main Street, Apt 4	·	Springfield	Ill.	62701

After

street	unit	city	region	postal_code
123 Main St	·	Springfield	IL	62701
123 Main St	·	Springfield	IL	62701
123 Main St	Apt 4	Springfield	IL	62701

The first two rows now match. The third names a specific apartment, which moves to its own column, and stays distinct.

Addresses are typed by hand, and street types, directions, and unit numbers each have several accepted short forms.
Forms differ in how they divide an address, so the unit number lands in the street line on one and in a second line on another.
Postal conventions vary by country, and a layout built for one country is used for all.

Counting households or sites gives too many, because one place appears under several spellings.
Mail is returned or delayed, and the same address can receive several copies.
Matching records across lists by address finds only the rows that happen to be written alike.

Group by postal code and read the street values inside each group side by side.
List distinct values of the region column. Full names, short codes, and dotted abbreviations together are a sign.
Search for unit words such as apt, suite, and unit to see where they sit and how they are spelled.

Put each part in its own column: street line, unit, city, region, postal code, country.
Choose one form for street types, directions, and regions, and apply it throughout.
Check against an authoritative postal reference where one exists for the country. Formatting alone cannot tell you an address is real.
Do not merge two rows because the street matches. A different unit is a different address.

Name variantsThe same person or organization appears under several spellings, orders, and abbreviations.
Near-duplicate recordsTwo rows describe the same real thing but differ enough that an exact comparison misses them.
Identifiers that lost their leading zerosA code that starts with zero, such as a postal code or account number, was read as a number and the zeros were dropped.
Case example: Finance teamsA vendor list that paid the same invoice twice