Overview

Teaching: 10 min
Exercises: 10 min

Questions

• 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.

Lesson

Filtering

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.

1. Click the down arrow next to respondent_roof_type > Text filter. A respondent_roof_type facet will appear on the left margin.
2. Type in mabat and press return. There are 58 matching rows of the original 131 rows (and these rows are selected for the subsequent steps).
3. At the top, change the view to Show 50 rows. This way you will see most of the matching rows.

Exercise

1. What roof types are selected by this procedure?
2. How would you restrict this to only one of the roof types?

Solution

1. Do Facet > Text facet on the respondent_roof_type column after filtering. This will show that two names match your filter criteria. They are mabatipitched and mabatisloping.
2. To restrict to only one of these two roof types, you could include more letters in your filter.

Excluding entries

In addition to the simple text filtering we used above, another way to narrow our filter is to include and/or exclude entries in a facet. You will see the include or exclude options if you hover over the name in the facet window.

If you still have your facet for respondent_roof_type, you can use it, or use drop-down menu > Facet > 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.

Exercise

Use include / exclude to select only entries from one of these two roof types.

Solution

1. 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 edit and include.
2. Click include. This will explicitly include this roof type, and exclude others that are not explicitly included. Notice that the option now changes to exclude.
3. Click include and exclude on 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.

Sort

You can sort the data by a column by using the drop-down menu in that column. There you can sort by text, numbers, dates or booleans (TRUE or FALSE values). You can also specify what order to put Blanks and Errors 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 sorting.

Exercise

Sort the data by gps_Altitude. Do you think the first few entries may have incorrect altitudes?.

Solution

In the gps:Altitude column, select Sort... > numbers and select 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 > Sort... - This option enables you to modify your original sort.
• Sort > Reverse - This option allows you to reverse the order of the sort.
• 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 > Sort > Remove sort, that column is removed from your multiple sort. If it is the only column sorted, then data reverts to its original order.

Exercise

We discovered in an earlier lesson that the value for one of the village entries 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?

1. Sort on gps_Longitude as a number with the largest first.
2. Add a sort on gps_Lattitude as a number with the largest first.
3. Using the drop down arrow on the village column, select Edit column > Move column to end. This will allow you to compare village names with GPS coordinates.
4. Scroll through the entries until you find village 49. Can you tell from it’s GPS coordinates which village it belong to?
5. Now sort only by interview_date as date. Move the village column to the start of the table. Does the row where village is 49 group with one particular village? Is it the same village as when comparing GPS coordinates?

Solution

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.

Key Points

• OpenRefine provides a way to sort and filter data without affecting the raw data.