REDCap Quirks

This page is for documenting quirks that appear in REDCap.


REDCap allows you to create fields for collecting dates, but it does not allow for collecting time zones associated with those dates. If you wish to record that information, you'll need to add a separate field to track it, like a multiple choice field.

Any date format that you specify is ONLY used for data entry through the web interface. Whenever you export data, any dates will be converted to year-month-day format. This includes dates in the data dictionary. For example, if you create a date field and specify a maximum and minimum date range for validation purposes, those dates will be in year-month-day format when you export the data dictionary, regardless of what format you chose.

Calculated Fields

Calculated fields in REDCap are buggy and should in general only be used for display purposes. Complex formulas in calculated fields cannot be relied upon to correctly calculate values. The reason for this is that calculated fields are not always updated when the data changes. For example, if you have a calculated field on a form X that depends on a value in form Y, the result of the calculation in form X will not be automatically updated if the value of the field in form Y changes. So if you were to export data after changing values in form Y, the calculated field value in form X may not be valid any longer.

REDCap has a data quality tool for re-calculating all calculated fields, but this also is not perfect. Normally, the formulas in calculated fields are computed using Javascript as data entry in your browser occurs. Unfortunately, the data quality tool does not use the same algorithm, and there are subtle differences in how the values are calculated. This is another reason why it's a bad idea to rely on REDCap's calculated fields for scientific analysis. If you must use them, make sure you are verifying them.

Exporting Data

If you have a large project in REDCap, be aware that exporting all of your data at once is problematic. REDCap runs into problems during the data export process due to time reasons. Because of the way REDCap stores data internally, exporting large amounts of data can be a computationally expensive task. Because the web server that REDCap runs on has a timeout, if the data export takes too long, it will simply stop before it can complete. This can be dependent on the server load, so if you are having trouble exporting data, you may have more luck during off times (like after normal work hours or on the weekend).

There are a few ways to get around this. You can export your data in parts by selecting forms in batches. If you know how to use the REDCap API, you can also split up the export by records programmatically.

Data Dictionary

REDCap's data dictionary file has some quirks of its own. If you export the data dictionary in CSV format through the web interface, you'll notice that some of the columns are being reused for multiple things. For example, there's a column with a header of "Choices, Calculations, OR Slider Labels". For each row in the dictionary, the contents of this column will have different meanings depending on the type of the field for that row. Another such column is labeled as "Text Validation Type OR Show Slider Number".

If you export the data dictionary through the API's "metadata" action, the column headers are not the same as if you exported it via the web interface. Instead, the column headers are all lowercase, and words are separated by underscores instead of spaces. For example, instead of "Choices, Calculations, OR Slider Labels", you get "select_choices_or_calculations". Note that any reference to slider labels is missing in the column header.

-- JeremyStephens - 26 Jan 2017
Topic revision: r1 - 26 Jan 2017, JeremyStephens

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