Use of Self-Generated Identification Codes 

for Data Linkages in Highly Clustered 

Samples Involving Youth Populations

Authors: 

Chidinma Nwankwo, Tiffany Waits, Avery Hennigar, Lourdes 

Fernandez, Amy Harris, William Waddell, Sam Bonelli, and Elizabeth 

Cavadel

Affiliation: Mathematica
Presenters

Tiffany Waits 

(She/Her)

Principal Survey Researcher

Human Services, Mathematica

Chidinma (Chichi) 

Nwankwo 

(She/Her)

Researcher Analyst

Human Services, Mathematica

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Study Challenge: 

How do we link pre and post surveys at the individual 

level without using personally identifiable information?

• Need to link pre and post surveys to measure change over time

• No PII to allow for linking surveys

• Youth in schools and out-of-school settings 

• Multi-mode survey based on program preference (paper or web)

• Youth highly clustered (grouped) into classes
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⁄ Self-Generated Identification Codes (SGICs)

⁄ Researchers have used questions about 

respondents' characteristics to match data 

(Kristijansson et al., 2013) and to link responses 

between different time points (Calatrava et al., 

2022).

Solution:

Ask a set of questions about respondent’s characteristics

on pre and post surveys to use for linking
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SGIC questions were selected from literature

- First name (first and third letters)

- Last name (third letter)

- Date of birth (second digit of the day) 

- Mother’s first name (second and 

fourth letters)

- Father’s first name (third letter)

- Paternal grandmother’s first name 

(second letter) 

- Color of respondent’s eyes (choosing 

the initial of the color from a range of 

options). 

Common examples include variations on the following:

Galanti et al., 2007 
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Study Application: 

Test SGIC method for linking in highly clustered 

settings with youth population

⁄ Apply method to youth population

⁄ In highly clustered settings of classrooms

⁄ Test 8 different questions across 2 survey versions

- 1 question was the same on both versions; a second question was the same with 

different response options

⁄ Each student received the same version survey and exact same 

questions at pre and post
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2 different sets of SGIC questions 

tested on 2 forms

⁄ SGIC Question Set 1 ⁄ SGIC Question Set 2
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How do the SGIC questions create a 

“unique code” for participants?

SGIC questions 

from Form A

Example of 

student response

Numerical value of 

response

Site and Classroom 

Code

Final SGIC

What is the first letter of 

your first name?

T 20

On what day of the 

month were you born?

6 6

What is the second 

letter of your last name?

A 1

What is the first letter of 

the street where you 

live?

R 18

What is the first letter of 

your mother’s first 

name?

N 14

SGIC CREATED 20611814 S1C4 S1C120611814
Findings
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⁄ Pre and post survey data 

linked for 83% of the students. 

- To match, students needed to 

answer 3, 4, or 5 questions out of 5 

the same between pre and post.

⁄ 55% of the students matched 

on all 5 questions.

⁄ Some questions worked better 

than others.

⁄ Missing data reduced ability to 

match.
55% students matched on all 

5 SGIC questions; 18% matched on 2 or less

SGIC questions Number of students %

Total matched

Matched on all 5 questions 28 55%

Matched on 4 of 5 questions 11 22%

Matched on 3 of 5 questions 3 6%

Matched on 2 of 5 questions 3 6%

Matched on 1 of 5 questions 3 6%

Matched on 0 questions 3 6%

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83%
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The number of surveys matched (> 3 identical 

SGIC questions) and the amount of missing 

data varied by classroom

Class Question 

Set

Pre-program 

survey 

respondents

Post-program 

survey

respondents

Total students 

matched pre 

to post

Number 

missing Match rate

Class 1 Set 1 21 16 12/16 5 75%

Class 2 Set 2 16 9 7/9 7 77%

Class 3 Set 2 13 13 13/13 0 100%

Class 4 Set 1 16 13 10/13 3 77%

Total 66 51 42/51 15 82%

0
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Some SGIC questions had higher match 

rates than others

SGIC question Total cases Total cases 

matched

Pre/Post survey 

match rate

In what month were you born? 22 21 95%

What is the first letter of your first name? (on form A and B) 48 44 92%

What color are your eyes? If they are more than one color, pick the color 

they are closest to. 

22 21 91%

On what day of the month were you born? 26 23 88%

What is the first letter of your middle name? Note included an option for 

No Middle Name

22 18 82%

What is the first letter of your mother’s first name? 26 21 81%

What is the second letter of your last name? Please select the response 

that includes the letter. (on form A and B) 

48 39 81%

What is the first letter of the name of the street where you live? 26 20 77%
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What we learned:

Using SGICs to match individual data from two 

points is a viable option

⁄ A clustered population improves the 

percentage of surveys that can be 

matched from pre to post.

⁄ Missing data from pre or post cannot 

be tracked or accounted for and 

reduces matching

⁄ The questions matter
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What next?

Conduct validity check to confirm matches and track 

missing surveys 

Assess differences by survey mode

Explore variations in question formatting and code 

generation options

Develop and test SGIC questions for different populations 

and settings
Thank you!

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