This document discusses various incentive mechanisms for encouraging referrals in crowdsourcing applications. It examines mechanisms that provide compensation for direct referrals as well as indirect or recursive referrals. Field experiments were conducted comparing mechanisms that provide 1, 3, or 1.5 points for direct referrals versus a control with no points. The experiments found that mechanisms providing compensation for referrals increased the number of referred sign-ups and actions. The document also analyzes different mechanisms through game theoretic lenses like the Shapley value to evaluate fairness and balanced contributions between participants.

1. Referrals
Victor Naroditskiy
joint work with
Talal Rahwan, Tomasz Michalak
Seb Stein, Long Tran-Thanh, Mirco Tonin, Michael Vlassopoulos
Iyad Rahwan, Manuel Cebrian
Nick Jennings

2. Crowdsourced Manhunt (Before)
2

3. Can you find me in
New York today?
Crowdsourced Manhunt (Now)
3
Tag Challenge

4. ...and four of my friends in
DC, London, Stockholm, and Bratislava.
4

5. Our team found 3 out of 5 and won.
5

6. ...despite our team’s Twitter and
Facebook gaining very little attention
6

7. Told about the challenge
and our team to online
media before the
competition
which led to posts on Slashdot and other forums.
7

8. When deciding to participate, people
considered credibility of our team.
University
affiliations
Winner of the
Red Balloon
challenge
Mentions on
CNET, ZDNET
Twitter and
Facebook accounts
with updates
8

9. Fair incentives encouraged participation and
referral of friends.
1
2
3
F
$1 for the first 2,000
referrals.
Finder receives
$500.
Referrer of the
finder receives
$100.
$5,000 prize distributed
among participants.
9

10. More details in
[Computer 2013]
[PLoS ONE 2013]
10

11. DARPA Red Balloon Challenge
11

12. MIT Mechanism:
½-split contract
1
2
3
F 500
62.5
250
125
recursive
12
Theoretical
justification for this
mechanism.
[PLoS ONE 2012]

13. Other Ways to Motivate Recruitment
1
2
3
F 800
0
200
0
Invite people
who may know
the answer.
finder + referrer
13
Our strategy in
the Tag Challenge.

14. Other Ways to Motivate Recruitment
1
2
3
F 800
0
100
100
Invite people
who may invite
many friends.
2 x finder + referrer
14

15. Successful paths are short (fewer than 2 referrals).
1
2
3
F 500
62.5
250
125
1
2
3
F 800
0
200
0
1
2
3
F 800
0
100
100
recursive finder + referrer 2 x finder + referrer
Use simpler non-recursive mechanisms? 15

16. How to Incentivize Referrals?
Field experiment
16

17. Incentive Mechanisms
• 1 point for each referral
• 3 points for each referral
• 1 point per referral, ½ point for indirect referral, ¼
point for... (recursive)
• Control: no points for referrals.
17

18. 18
Facebook posts bring 22% of total donations.

19. 19

20. 20
Treatments

21. Data
• 416 observations
• Looked at differences in
– referred sign-ups
– referred visits
– referral actions (clicking to tweet or post on fb).
21

22. Probability of Generating One or More
Visit

23. Probability of Generating One or More
Signup

24. Next field experiment
• Easier to attract participants (mTurk or in
partnership with a high-traffic site)
• Gamification
– competition to collect more points
• Social recognition
– points result in the user being acknowledged.
24

25. Financial incentives gone wrong
Map AEDs in Philadelphia.
Most of the submissions by 2
participants who went door-to-door.
No monetary incentives.
Successful crowdsourcing
of AEDs in the Netherlands.
“Crowding out” altruistic
spirit. 25

26. Theory: referrals and fairness
26

27. For every friend who joins and installs
Dropbox on their computer, we'll give you
both 500 MB of bonus space.
27
1 referral is worth 1GB.

28. 1
2
3
1/2
1/2
Recursive
+ 1/4
Invitee not
compensated.
30

29. 1
2
3
1/2
1/2
1/2
Dropbox
+ 1/2
No compensation
for indirect
referrals.
31

30. Shapley Value
32
Balanced Contributions:
Your reward from my presence equals
my reward from your presence.
Efficiency:
Sum of rewards = total value.

31. 1
3
Reward: Root-dependent
33
v = 0
1
2
3
v = 2
2
3
v = 0
0
1
2
v = 1
00
0

32. 1
2
3
1/2
1/3
1/2
Shapley
+ 1/3
+ 1/3
34

33. 1
2
3
1/2
1/2
1/2
Symmetric
+ 1/2
+ 1/4
35
+ 1/4

34. Symmetric
36
Receive sd for every neighbor at distance d
Dropbox: s1 = 1/2, s2 = 0, s3 = 0 ...
Satisfies balanced contributions!

35. Symmetric vs Geometric
37
Setting sd = (1/2)d makes Symmetric similar to Geometric
Compensation for descendants is the same under
Geometric and Symmetric, but Symmetric also
compensates for ancestors
Geometric mechanisms do not satisfy balanced
contribution

36. 1
3
Reward: Root-independent
38
1
2
3
v = sum of rewards
2
3
s1
1
2
s1s2
s2
Symmetric vs Geometric

37. Recursive Shapley
No balanced
contributions.
Balanced
Contributions:
Root-dependent
39
1
2
3
1/2
1/3
1/2 + 1/3
+ 1/3 1/2
1/2
1/2 + 1/2
+ 1/4
+ 1/4
Symmetric
Balanced
Contributions
1/2
0
1/2
+ 1/4

38. Recursive Shapley
40
1
2
3
833
333
833
600
600
800
Symmetric
1200
0
800
Normalized to Distribute 2GB

39. Trees
41
Referral structure:
Under all mechanisms, an ancestor is compensated for at
least as many nodes as his descendent.
1
2 3
4 5 6 7
8 9 10

40. Thank you
42