A presentation by Dr. Robin Upton (2006-01-28) Available for download at www.altruists.org/ae14

Attribution – NonCommercial - ShareAlike

www.altruists.org

Altruistic Economics

AE18: The Imaginary Friends Test

Introduction to Imaginary Friends

Recommended Pre-requisite: AE10: Keeping Score

v1.0

2

Introduction to Imaginary Friends

. F2F Network Growth

• Altruistic Economics is a decentralised system, so

users have a lot of autonomy... .

• Rewarding such Imaginary Friends would encourage deception.

the chance to create Imaginary Friends:

Artificial

ConstructReal Individual

User

, such as

3

F2F Network Growth

Sybil Attacks

The Friend2Friend network grows organically, over real social relationships.

However,like WWW, there is no centre, noone in charge, no list of members.

Each new member creates an online server and is linked in by friends.

The idea is, everyone has one online server.

4

Sybil Attacks

The ‘Imaginary Friends’ Test

A Sybil Attack is an attempt to defraud an online system by using fake identities.

‘Sybil’ was the first person diagnosed with so-called ‘Multiple Personality Disorder’.

5

The ‘Imaginary Friends’ Test“Does it help to create ‘imaginary friends’?”

. Media Sharing

2. Imaginary Friends are a disadvantage:

1. Imaginary Friends make no difference:

PASS: No Incentive to Cheat

3. Imaginary Friends are an advantage:

FAIL: Incentive to Cheat

6

Media SharingMEDIA SHARING is a project to network people who review files and then benefit from one another’s opinions.

Basic Framework

BirbalAysha

Aysha wants an opinion about the film ‘Chinese Whispers’…

I value your opinion…

Chris“Chinese Whispers”

I value your opinion…

How to quantify this process... ?

I know this film

7

Basic Framework• Individuals make 2 sorts of evaluations:

Combining Recommendations

1) ‘I value your opinion’ is quantified as V: 0<=V<=12) ‘I rate this File’ is quantified as R: 0<=R<=1

• Friends of friends’ opinions are valued multiplicatively

So Aysha has one recommendation: Value=0.12 Rating=0.8

“Chinese Whispers”

BirbalAysha I value your opinion… 0.3 ChrisI value your

opinion… 0.4 I rate this file… 0.8

• Individuals who don’t rate a file defer to their friends

Assume:

8

Combining Recommendations

Applying The Test

Ratings are averaged according to value, to give a single figure.

So, if Aysha has 4 recommendations: Value=0.15 Rating=0.6Value=0.8 Rating=0.5Value=0.08 Rating=0.6Value=0.12 Rating=0.8

0.15×0.6+ 0.8×0.5+ 0.08×0.6+ 0.12×0.8

0.15+0.8+0.08+0.12

Weighted average = ≈ 0.55

Note the high value of the 2nd rating, which means it dominates the others.

9

Applying The Test

A Problem

“Chinese Whispers”

MBogoN

ChrisMBogo2

Rating = R

I value your opinion… 1.0 MBogo1Value=VB

I value your opinion… 1.0I value your opinion… 1.0

Rating = R

Rating = R

…

What if Chris made Imaginary Friends to rate the file for him?

Value=VB Rating=RValue=VB Rating=R

…{N separate ratings have Overall value = NVB

10

A Problem

A Modification

Imaginary friends can increase an individuals’ impact, so the algorithm fails the imaginary friends test.

Chris could use his Imaginary Friends to dominate the other ratings.

So, if Aysha has 4 recommendations: Value=0.15 Rating=0.6Value=0.8 Rating=0.5Value=0.08 Rating=0.6Value=NVB Rating=R

If N is big enough, no one else’s ratings count!

11

A Modification

Summary of The ‘Imaginary Friends’ Test

To control the imaginary friends, we add a constraint to the model.

“Chinese Whispers”

MBogoN

ChrisMBogo2

Rating = R

V1 MBogo1Value=VB

vN

Rating = R

Rating = R

…

v2

…

N friends give Chris N ratings, with a value of ∑Vf.

f

Let’s require ∑Vf=1 f

12

Modified Algorithm Examined

Modified Algorithm Examined

So the modified algorithm passes the imaginary friends test.

With N>0 imaginary friends: As an individual:VB

Chris

“Chinese Whispers”

Rating = R

Single rating: Value=VB Rating=R

…} N such friendsMBogofRating = Rf

“Chinese Whispers”

ChrisVf

VB

imaginary friends confer no advantage.

Overall rating: Value= VB∑Vf Rating=∑VfRff f

Since ∑Vf=1, 1≥R,Rf≥0…f

13

Summary of The Imaginary Friends Test

Recommended Further Reading: AE19: Preventing Abusehttp://www.altruists.org/ae19

• Applying the Imaginary Friends Test helps deduce algorithms suitable for decentralised systems.

• Sybil Attacks are a feature of decentralised systems.

Creating Imaginary Friends

should not confer any advantage: