Page Comparison

Identifiers
The goal of this article is to assist users in understanding the different types of identifiers: 

• The first variable listed in your project is the unique identifier which links all data for a specific record across all instruments.
  • Do not use Protected Health Information (PHI) identifiers such as medical record number or date of birth or initials as the unique identifier, as it could be accidentally displayed if a URL is created. The medical record number or date of birth, if required, can be added as another field.
  • In Data Entry projects, you must define the unique identifier field. For projects where a survey is the first data collection instrument, it is automatically defined as the Participant ID. The Participant ID value is numeric and auto-increments starting with the highest value in the project. If no records exist, it will begin with '1'.
  • Users can define the unique ID for projects with surveys instead of using the participant_id by having the first data collection instrument as a data entry form (do NOT enable it as a survey).
• The optional secondary unique field may be defined as any field on the data collection instruments. The value for the field you specify will be displayed next to the Participant ID (for surveys) or next to your unique identifier when choosing an existing record/response. It will also appear at the top of the data entry page when viewing a record/response. Unlike the value of the primary unique identifier field, it will not be potentially visible in a URL.
  • The data values entered into the secondary unique field must also be unique. The system will not allow for duplicate entries and checks values entered in real time. If a duplicate value is entered, an error message will appear and the value must be changed to save/submit data entered on the data entry instrument.
  • Common secondary unique identifiers are medical record numbers, subject name, and subject birthdate.
• The redcap_survey_identifier is the identifier defined for surveys when utilizing the Participant Email Contact List and sending survey invitations from the system. The "Participant Identifier" is an optional field you can use to identify individual survey responses so that the participant doesn't have to enter any identifying information into the actual survey. This field is exported in the data set; the email address of the participant is not.

The unique identifier must be a 'text' field. In addition, please note that unique identifier values will be visible at the end of the URL – and likely cached in web browsers – as individual records are viewed or entered.
(Example URL: https://xxx.xxx.xxx/redcap/redcap_vx.x.x/data_entry.php?pid=xxx&p age=xxx&id=ID_VARIABLE_VALUE.)
Again, do not use Protected Health Information (PHI) Identifiers such as MRN or DOB+initials as the unique identifier. This is an additional precaution to preserve research participant confidentiality from displaying in the URL and becoming cached. 

Additional Considerations

Q: If the unique identifier is arbitrary to me, can the system auto-assign a unique value to each of my records? Yes. You can enable auto-numbering for naming new project records on the Project Setup > Enable optional modules and customizations page. This option will remove the ability for users to name new records manually and will instead provide a link that will auto-generate a new unique record value. The value is numeric and increments from the highest numeric record value in the project. If no records exist, it will begin with '1'.

Q: How can I set the default auto-numbering to start at a particular number such as 2000? You can disable auto-numbering and add the first record using the ID number as the start value. Once this record is saved, you can enable the auto-numbering customization, so subsequent records will be created starting after 2000.

Q: What's the difference between the unique identifier, secondary unique identifier and the redcap_survey_identifier?

• The first variable listed in your project is the unique identifier which links all data for a record.
• The secondary unique field

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• allows you to identify another unique identifying value of the field

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• (e.g, name, subject #, ). It will also appear at the top of the data entry page when viewing a record/response. Unlike the value of the primary unique identifier field, it will not be visible in the URL

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• .
• The redcap_survey_identifier is the identifier defined for surveys when utilizing the Participant Email Contact List and sending survey invitations from the system. The "Participant Identifier" is an optional field you can use to identify individual survey responses so that the participant doesn't have to enter any identifying information into the actual survey. This field is exported in the data set; the email address of the participant is not.

Q: If we wanted to get rid of our own IDs and just use auto-numbering, could we start at, say, 3000? 
 If you wanted to start the auto-numbering record ID at 3000, you would have to go in to an existing record (it is best to create a fake record to do this) and manually change the record number to 2999 so that the next assigned record # would be 3000. You will need to have "Rename Records" ability under your User Rights in order to do this. The hard stop rule for auto numbering is that it can't be done if you have a public survey. If the project has a public survey it has to have auto numbering.

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