OverviewTeaching: 10 min
Exercises: 0 minQuestions
What is OpenRefine useful for?Objectives
Describe OpenRefine’s uses and applications.
Differentiate data cleaning from data organization.
Experiment with OpenRefine’s user interface.
Locate helpful resources to learn more about OpenRefine.
Motivations for the OpenRefine Lesson
- Data is often very messy. OpenRefine provides a set of tools to allow you to identify and amend the messy data.
- It is important to know what you did to your data. Additionally, journals, granting agencies, and other institutions are requiring documentation of the steps you took when working with your data. With OpenRefine, you can capture all actions applied to your raw data and share them with your publication as supplemental material.
- All actions are easily reversed in OpenRefine.
- If you save your work it will be to a new file. OpenRefine always uses a copy of your data and does not modify your original dataset.
- Data cleaning steps often need repeating with multiple files. OpenRefine keeps track of all of your actions and allows them to be applied to different datasets.
- Some concepts such as clustering algorithms are quite complex, but OpenRefine makes it easy to introduce them, use them, and show their power.
- Open source (source on GitHub).
- A large growing community, from novice to expert, ready to help. See Getting Help section below.
- Works with large-ish datasets (100,000 rows). Can adjust memory allocation to accommodate larger datasets.
- OpenRefine always keeps your data private on your own computer until you choose to share it. It works by running a small server on your computer and using your web browser to interact with it, but your private data never leaves your computer unless you want it to.
Before we get started
Note: this is a Java program that runs on your machine (not in the cloud). It runs inside your browser, but no web connection is needed.
Follow the Setup instructions to install OpenRefine.
If after installation and running OpenRefine, it does not automatically open for you, point your browser at http://127.0.0.1:3333/ or http://localhost:3333 to launch the program.
Getting help for OpenRefine
You can find out a lot more about OpenRefine at http://openrefine.org and check out some great introductory videos.
These videos and others on OpenRefine can also be found on YouTube by searching under ‘OpenRefine’. There is an official forum that can answer a lot of beginner questions and problems. Information can also be found on StackOverflow where you can find a lot of help. As with other programs of this type, OpenRefine libraries are available too, where you can find a script you need and copy it into your OpenRefine instance to run it on your dataset.
OpenRefine is a powerful, free, and open source tool that can be used for data cleaning.
OpenRefine will automatically track any steps allowing you to backtrack as needed and providing a record of all work done.
Working with OpenRefine
OverviewTeaching: 15 min
Exercises: 20 minQuestions
How can we bring our data into OpenRefine?
How can we sort and summarize our data?
How can we find and correct errors in our raw data?Objectives
Create a new OpenRefine project from a CSV file.
Understand potential problems with file headers.
Use facets to summarize data from a column.
Use clustering to detect possible typing errors.
Understand that there are different clustering algorithms which might give different results.
Employ drop-downs to remove white spaces from cells.
Manipulate data using previous steps with undo/redo.
Creating a new OpenRefine project
If you have not started OpenRefine yet, follow the Setup instructions before continuing.
OpenRefine can import a variety of file types, including tab separated (
comma separated (
csv), Excel (
xlsx), JSON, XML, RDF as XML, and
Google Spreadsheets. See the OpenRefine Create a Project by Importing Data
for more information.
In this first step, we’ll browse our computer to the sample data file for this lesson. In this case, we will be using data obtained from interviews of farmers in two countries in eastern sub-Saharan Africa (Mozambique and Tanzania). If you haven’t yet downloaded the data, see the instructions on downloading the data in Setup.
Once OpenRefine is launched in your browser, the left margin has options to
Open Project, or
Import Project. Here we will create a
Create Projectand select
Get data from
Choose Filesand select the file
SAFI_openrefine.csvthat you downloaded in the setup step. Click
Openor double-click on the filename.
Next>>under the browse button to upload the data into OpenRefine.
OpenRefine gives you a preview - a chance to show you it understood the file. If, for example, your file was really tab-delimited, the preview might look strange. You would then choose the correct separator in the box shown and click
Update Preview(middle right). If this is the wrong file, click
<<Start Over(upper left). There are also options to indicate whether the dataset has column headers included and whether OpenRefine should skip a number of rows before reading the data.
- If all looks well, click
Create Project>>(upper right).
Note that at step 1, you could upload data in a standard form from a web
address by selecting
Get data from
Web Addresses (URLs).
The URLs must point to data in a file type that OpenRefine understands, just
like the types that you could upload.
Instead of downloading the dataset file as you did during
setup and uploading it from your computer,
you could have submitted its URL here.
Fully understanding this functionality is out of scope for this lesson.
The OpenRefine manual’s section on importing from Web addresses
Exploring data by applying multiple filters
Facets are one of the most useful features of OpenRefine and can help both get an overview of the data in a project as well as help you bring more consistency to the data. OpenRefine supports faceted browsing as a mechanism for
- seeing a big picture of your data, and
- filtering down to just the subset of rows that you want to change in bulk.
A ‘Facet’ groups all the like values that appear in a column, and then allows you to filter the data by these values and edit values across many records at the same time.
One type of Facet is called a ‘Text facet’. This groups all the identical text values in a column and lists each value with the number of records it appears in. The facet information always appears in the left hand panel in the OpenRefine interface.
Here we will use faceting to look for potential errors in data entry in the
- Scroll over to the
- Click the down arrow and choose
- In the left panel, you’ll now see a box containing every unique value in the
villagecolumn along with a number representing how many times that value occurs in the column.
- Try sorting this facet by name and by count. Do you notice any problems with the data? What are they?
- Hover the mouse over one of the names in the
Facetlist. You should see that you have an
- You could use this to fix an error immediately, and OpenRefine will ask whether you want to make the same correction to every value it finds like that one. But OpenRefine offers even better ways to find and fix these errors, which we’ll use instead. We’ll learn about these when we talk about clustering.
Chirdozois likely a mis-entry of
Rucais likely a mis-entry of
Ruaca - Nhamuendaand
Ruaca-Nhamuendarefer to the same place (differ only by spaces around the hyphen). You might also wonder if both of these are the same as
Ruaca. We will see how to correct these misspelled and mistyped entries in a later exercise.
- The entry
49is almost certainly an error but you will not be able to fix it by reference to other data.
Using faceting, find out how many different
interview_datevalues there are in the survey results.
Is the column formatted as Text or Date?
Use faceting to produce a timeline display for
interview_date. You will need to use
To dateto convert this column to dates.
During what period were most of the interviews collected?
For the column
Text facet. A box will appear in the left panel showing that there are 19 unique entries in this column. By default, the column
interview_dateis formatted as Text. You can change the format by doing
Notice the the values in the column turn green. Doing
Timeline facetcreates a box in the left panel that shows a histogram of the number of entries for each date.
Most of the data was collected in November of 2016.
More on Facets
As well as ‘Text facets’ Refine also supports a range of other types of facet. These include:
- Numeric facets
- Timeline facets (for dates)
- Custom facets
- Scatterplot facets
Numeric and Scatterplot facets display graphs instead of lists of values. The numeric facet graph includes ‘drag and drop’ controls you can use to set a start and end range to filter the data displayed. These facets are explored further in Examining Numbers in OpenRefine
Custom facets are a range of different types of facets. Some of the default custom facets are:
- Word facet - this breaks down text into words and counts the number of records each word appears in
- Duplicates facet - this results in a binary facet of ‘true’ or ‘false’. Rows appear in the ‘true’ facet if the value in the selected column is an exact match for a value in the same column in another row
- Text length facet - creates a numeric facet based on the length (number of characters) of the text in each row for the selected column. This can be useful for spotting incorrect or unusual data in a field where specific lengths are expected (e.g. if the values are expected to be years, any row with a text length more than 4 for that column is likely to be incorrect)
- Facet by blank - a binary facet of ‘true’ or ‘false’. Rows appear in the ‘true’ facet if they have no data present in that column. This is useful when looking for rows missing key data.
Using clustering to detect possible typing errors
In OpenRefine, clustering means “finding groups of different values that might
be alternative representations of the same thing”. For example, the two strings
New York and
new york are very likely to refer to the same concept and just
have capitalization differences. Likewise,
Godel probably refer
to the same person. Clustering is a very powerful tool for cleaning datasets
which contain misspelled or mistyped entries. OpenRefine has several clustering
algorithms built in. Experiment with them, and learn more about these
algorithms and how they work.
- In the
villageText Facet we created in the step above, click the
- In the resulting pop-up window, you can change the
Keying Function. Try different combinations to see what different mergers of values are suggested.
- Select the
key collisionmethod and
metaphone3keying function. It should identify two clusters.
- Click the
Merge?box beside each cluster, then click
Merge Selected and Reclusterto apply the corrections to the dataset.
- Try selecting different
Keying Functionsagain, to see what new merges are suggested.
- You should find that using the default settings, no more clusters are found,
for example to merge
Chirodzo. (Note that the
nearest neighbormethod with
radius≥ 4, and
block chars≤ 4 will find these clusters, as well as other settings with
- To merge these values we will hover over them in the village text facet,
select edit, and manually change the names. Change
Ruaca. You should now have four clusters:
Important: If you
Merge using a different method or keying function, or more
times than described in the instructions above, your solutions for later
exercises will not be the same as shown in those exercise solutions.
Different clustering algorithms
The technical details of how the different clustering algorithms work can be found at the link below.
The data in the
items_owned column is a set of items in a list. The list is
in square brackets and each item is in single quotes. Before we split the list
into individual items in the next section, we first want to remove the brackets
and the quotes.
- Click the down arrow at the top of the
This will open up a window into which you can type a GREL expression. GREL stands for General Refine Expression Language.
First we will remove all of the left square brackets (
[). In the Expression box type
value.replace("[", "")and click
What the expression means is this: Take the
valuein each cell in the selected column and replace all of the “[” with “” (i.e. nothing - delete).
OK. You should see in the
items_ownedcolumn that there are no longer any left square brackets.
Use this same strategy to remove the single quote marks (
'), the right square brackets (
]), and spaces from the
value.replace(" ", "")You should now have a list of items separated by semi-colons (
Now that we have cleaned out extraneous characters from our
column, we can use a text facet to see which items were commonly owned or
rarely owned by the interview respondents.
- Click the down arrow at the top of the
Custom text facet...
- In the
You should now see a new text facet box in the left-hand pane.
Which two items are the most commonly owned? Which are the two least commonly owned?
count. The most commonly owned items are mobile phone and radio, the least commonly owned are cars and computers.
Perform the same clean up steps and customized text faceting for the
months_lack_foodcolumn. Which month(s) were farmers more likely to lack food?
All four cleaning steps can be performed by combining
.replacestatements. The command is:
value.replace("[", "").replace("]", "").replace(" ", "").replace("'", "")This can also be done in four separate steps if preferred. November was the most common month for respondents to lack food.
Perform the same clean up steps for the
no_food_mitigationcolumns. Hint: To reuse a GREL command, click the
Historytab and then click
Reusenext to the command you would like to apply to that column.
Using undo and redo
It’s common while exploring and cleaning a dataset to discover after you’ve
made a change that you really should have done something else first. OpenRefine
Redo operations to make this easy.
- Click where it says
Undo / Redoon the left side of the screen. All the changes you have made so far are listed here.
- Click on the step that you want to go back to, in this case go back several steps to before you had done any text transformation.
- Visually confirm that those columns now contain the special characters that we had removed previously.
- Notice that you can still click on the later steps to
Redothe actions. Before moving on to the next lesson, redo all the steps in your analysis so that all of the columns you modified are lacking in square brackets, spaces, and single quotes.
Trim Leading and Trailing Whitespace
Words with spaces at the beginning or end are particularly hard for we humans to tell from strings without, but the blank characters will make a difference to the computer. We usually want to remove these. As of version 3.4 of OpenRefine, the option to trim leading and trailing whitespaces is present at the moment of importing the data (see image at the top of this page).
If you unchecked that box when importing data, or if leading or trailing whitespaces were introduced while splitting columns, or other operations, OpenRefine also provides a tool to remove blank characters from the beginning and end of any entries that have them.
- Edit the
villageon the first row to introduce a space at the end, set to
- Create a new text facet for the
villagecolumn. You should now see two different entries for
God, one of those has a trailing whitespace.
- To remove the whitespace, choose
Trim leading and trailing whitespace.
- You should now see only four choices in your text facet again.
OpenRefine can import a variety of file types.
OpenRefine can be used to explore data using filters.
Clustering in OpenRefine can help to identify different values that might mean the same thing.
OpenRefine can transform the values of a column.
Filtering and Sorting with OpenRefine
OverviewTeaching: 10 min
Exercises: 10 minQuestions
How can we select only a subset of our data to work with?
How can we sort our data?Objectives
Filter to a subset of rows by text filter or include/exclude.
Sort table by a column.
Sort by multiple columns.
There are many entries in our data table. We can filter it to work on a subset of the data in the list for the next set of operations. Please ensure you perform this step to save time during the class.
- Click the down arrow next to
Text filter. A
respondent_roof_typefacet will appear on the left margin.
- Type in
mabatand press return. There are 58 matching rows of the original 131 rows (and these rows are selected for the subsequent steps).
- At the top, change the view to
rows. This way you will see most of the matching rows.
- What roof types are selected by this procedure?
- How would you restrict this to only one of the roof types?
Text faceton the
respondent_roof_typecolumn after filtering. This will show that two names match your filter criteria. They are
- To restrict to only one of these two roof types, you could include more letters in your filter.
In addition to the simple text filtering we used above, another way to narrow
our filter is to
exclude entries in a facet. You will see
exclude options if you hover over the name in the facet
If you still have your facet for
respondent_roof_type, you can use it, or use
drop-down menu >
Text facet to create a new facet. Only the entries
with names that agree with your
Text filter will be included in this facet.
Faceting and filtering look very similar. A good distinction is that faceting gives you an overview description of all of the data that is currently selected, while filtering allows you to select a subset of your data for analysis.
include / excludeto select only entries from one of these two roof types.
- In the facet (left margin), click on one of the names, such as
mabatisloping. Notice that when you click on the name, or hover over it, there are entries to the right for
include. This will explicitly include this roof type, and exclude others that are not explicitly included. Notice that the option now changes to
excludeon the other roof type and notice how the two entries appear and disappear from the table.
Remove the filter before moving on so that you again have the full dataset of 131 records.
You can sort the data by a column by using the drop-down menu in that column.
There you can sort by
FALSE values). You can also specify what order to put
in the sorted results.
If this is your first time sorting this table, then the drop-down menu for the
selected column shows
Sort.... Select what you would like to sort by (such as
numbers). Additional options will then appear for you to fine-tune your
Sort the data by
gps_Altitude. Do you think the first few entries may have incorrect altitudes?
smallest first. The first few values are all 0. The altitudes are more likely ‘missing’ than incorrect. The survey is delivered by Smartphone with the gps information added automatically by the app. The lack of an altitude value suggests that the smartphone was unable to provide it and it defaulted to 0.
If you try to re-sort a column that you have already used, the drop-down menu
changes slightly, to >
Sort without the
..., to remind you that you have
already used this column. It will give you additional options:
Sort...- This option enables you to modify your original sort.
Reverse- This option allows you to reverse the order of the sort.
Remove sort- This option allows you to undo your sort.
Sorting by multiple columns
You can sort by multiple columns by performing sort on additional columns. The
sort will depend on the order in which you select columns to sort. To restart
the sorting process with a particular column, check the
sort by this column
alone box in the
Sort pop-up menu.
If you go back to one of the already sorted columns and select >
Remove sort, that column is removed from your multiple sort. If it is the
only column sorted, then data reverts to its original order.
We discovered in an earlier lesson that the value for one of the
villageentries was given as 49. This is clearly wrong. By looking at the GPS coordinates for the entries of the other villages can we decide what village the data in that column was collected from?
- Sort on
gps_Latitudeas a number with the smallest first.
- Add a sort on
gps_Longitudeas a number with the smallest first.
- Using the drop down arrow on the
Move column to end. This will allow you to compare village names with GPS coordinates.
- Scroll through the entries until you find village
49. Can you tell from it’s GPS coordinates which village it belong to?
- Now sort only by
interview_dateas date. Move the
villagecolumn to the start of the table. Does the row where village is
49group with one particular village? Is it the same village as when comparing GPS coordinates?
The interview data for that row is in a small cluster of Chirodzo interviews when sorting by GPS coordinates. When sorting by interview date, it is also with Chirodzo interviews. In fact, only Chirodzo had interviews conducted on that date.
Perform a text facet on the
village column and change
49 to the village
name that was determined in the previous exercise. You should now have only
three village names.
OpenRefine provides a way to sort and filter data without affecting the raw data.
Examining Numbers in OpenRefine
OverviewTeaching: 10 min
Exercises: 10 minQuestions
How can we convert a column from one data type to another?
How can we find non-numeric values in a column that should contain numbers?Objectives
Transform a text column into a number column.
Identify and modify non-numeric values in a numeric column using facets.
When a table is imported into OpenRefine, all columns are treated as containing
text values. We saw earlier how we can sort column values as numbers, but this
does not change the cells in a column from text to numbers. Rather, this
interprets the values as numbers for the purposes of sorting but keeps the
underlying data type as is. We can, however, transform columns from text to
other data types (e.g. number or date) using the
Edit cells >
transforms feature. Here we will experiment changing columns to numbers and
see what additional capabilities that grants us.
Be sure to remove any
Text filter facets you have enabled from the left panel
so that we can examine our whole dataset. You can remove an existing facet by
x in the upper left of that facet window.
To transform cells in the
years_farm column to numbers, click the down arrow
for that column, then
Edit cells >
Common transforms… >
To number. You
will notice the
years_farm values change from left-justified to
right-justified, and black to green in color.
Transform three more columns,
buildings_in_compound, from text to numbers. Can all columns be transformed to numbers? - Try it with
Only observations that include only numerals (0-9) can be transformed to numbers. If you apply a number transformation to a column that doesn’t meet this criteria, and then click the
Undo / Redotab, you will see a step that starts with
Text transform on 0 cells. This means that the data in that column was not transformed.
Sometimes non-numeric values or blanks occur in a column where numbers are
expected. Such values may represent errors in data entry, and we want to find
them. We can do that with a
- For a column you transformed to numbers, edit one or two cells, replacing the numbers with text (such as
abc) or blank (no number or text). You will need to change the
textusing the drop-down menu.
- Use the column pulldown menu to apply a numeric facet to the column you edited. The facet will appear in the left panel.
- Notice that there are several checkboxes in this facet:
Error. Below these checkboxes are counts of the number of cells in each category. You should see checks for
Blankif you changed some values.
- Experiment with checking or unchecking these boxes to select subsets of your data.
When done examining the numeric data, remove this facet by clicking the
the upper left corner of its panel. Note that this does not undo the edits you
made to the cells in this column. Use the
Undo / Redo function to reverse
OpenRefine also provides ways to to examine and clean numerical data.
OverviewTeaching: 10 min
Exercises: 10 minQuestions
How can we document the data-cleaning steps we’ve applied to our data?
How can we apply these steps to additional data sets?Objectives
Describe how OpenRefine generates JSON code representing the work done in an analysis session.
Demonstrate ability to export this JSON code from OpenRefine.
Demonstrate ability to import a JSON code file to apply the analysis to another dataset.
How OpenRefine records what you have done
Saving your work as a script
Undo / Redosection, click
Extract..., and select the steps that you want to apply to other datasets by clicking the check boxes.
Copy the code from the right hand panel and paste it into a text editor (like NotePad on Windows or TextEdit on Mac). Make sure it saves as a plain text file. In TextEdit, do this by selecting
Make plain textand save the file as a
Importing a script to use against another dataset
Let’s practice running these steps on a new dataset. We’ll test this on an uncleaned version of the dataset we’ve been working with.
- Start a new project in OpenRefine using the messy dataset you downloaded before. Give the project a new name.
- Click the
Undo / Redotab >
Applyand paste in the contents of
.txtfile with the JSON code.
Perform operations. The dataset should now be the same as your other cleaned dataset.
For convenience, we used the same dataset. In reality you could use this process to clean related datasets. For example, data that you had collected over different fieldwork periods or data that was collected by different researchers (provided everyone uses the same column headings). The data in this file was generated from an eSurvey system with the actual survey being delivered centrally to a smartphone, so the column headings are pretty much guaranteed to be the same.
All changes are being tracked in OpenRefine, and this information can be used for scripts for future analyses or reproducing an analysis.
Exporting and Saving Data from OpenRefine
OverviewTeaching: 10 min
Exercises: 5 minQuestions
How can we save and export our cleaned data from OpenRefine?Objectives
Save an OpenRefine project.
Export cleaned data from an OpenRefine project.
Saving and Exporting a Project
In OpenRefine you can save or export the project. This means you’re saving the data and all the information about the cleaning and data transformation steps you’ve done. Once you’ve saved a project, you can open it up again and be just where you stopped before.
By default OpenRefine is saving your project continuously. If you close OpenRefine and open it up again, you’ll see a list of your projects. You can click on any one of them to open it up again.
You can also export a project. This is helpful, for instance, if you wanted to send your raw data and cleaning steps to a collaborator, or share this information as a supplement to a publication.
- Click the
Exportbutton in the top right and select
OpenRefine project archive to file.
tar.gzfile will download to your default
Downloaddirectory. Depending on your browser you may have to confirm that you want to save the file. The
tar.gzextension tells you that this is a compressed file. The downloaded
tar.gzfile is actually a folder of files which have been compressed. Linux and Mac machines will have software installed to automatically expand this type of file when you double-click on it. For Windows based machines you may have to install a utility like ‘7-zip’ in order to expand the file and see the files in the folder.
- After you have expanded the file look at the files that appear in this folder. What files are here? What information do you think these files contain?
You should see:
historyfolder which contains a collection of
zipfiles. Each of these files itself contains a
change.txtfiles are the records of each individual transformation that you did to your data.
data.zipfile. When expanded, this
zipfile includes a file called
data.txtwhich is a copy of your raw data. You may also see other files.
You can import an existing project into OpenRefine by clicking
Open... in the
upper right >
Import Project and selecting the
tar.gz project file. This
project will include all of the raw data and cleaning steps that were part of
the original project.
Exporting Cleaned Data
You can also export just your cleaned data, rather than the entire project.
Exportin the top right and select the file type you want to export the data in.
csv) would be good choices.
- That file will be exported to your default
Downloaddirectory. That file can then be opened in a spreadsheet program or imported into programs like R or Python, which we’ll be discussing later in our workshop.
Remember from our lesson on Spreadsheets that using widely-supported,
non-proprietary file formats like
csv improves the ability of
yourself and others to use your data.
Cleaned data or entire projects can be exported from OpenRefine.
Projects can be shared with collaborators, enabling them to see, reproduce and check all data cleaning steps you performed.
Other Resources in OpenRefine
OverviewTeaching: 5 min
Exercises: 5 minQuestions
What other resources are available for working with OpenRefine?Objectives
Understand that there are many online resources available for more information on OpenRefine.
Identify other resources about OpenRefine.
Using online resources to get help with OpenRefine
OpenRefine is more than a simple data cleaning tool. People are using it for all sorts of activities. Here are some other resources that might prove useful.
OpenRefine has its own web site with documentation and a book:
- OpenRefine web site
- OpenRefine User Manual (the previous OpenRefine Wiki is being phased out)
- Using OpenRefine book by Ruben Verborgh, Max De Wilde and Aniket Sawant
- OpenRefine history from Wikipedia
In addition, see these other useful resources:
- Grateful Data is a fun site with many resources devoted to OpenRefine, including a nice tutorial.
- Margaret Heller shows how she uses OpenRefine for Measuring and Counting Impact in Repositories.
There are more advanced uses of OpenRefine, such as bringing in column or cell data using web locators (URLs or APIs). The links above can give you a start on your journey.
Visit one of these sites and share what you find with another person.
Other examples and resources online are good for learning more about OpenRefine.