Half-baked ideas: Demand-revealing referenda applied to Fedora features

For more half-baked ideas, see the ideas tag

Recently Lennart proposed dropping — and then unilaterally dropped a few days later — support for TCP wrappers in systemd. I haven’t used TCP wrappers for a long time, but there are some who do, and for those people dropping features like this provokes strong feelings. How should we conduct a survey or vote to decide what features to add or drop in software projects?

One way would be for all users [however that is defined] to have a vote. The problem with that is that a feature which few people use, but which really matters for those that use it would probably get dropped by a simple majority vote.

A better idea would be to use an economic system called a revealed preference. The idea is by asking people to risk their own money on the outcome of a vote, you hope to get a truer picture of their feelings. This technique also excludes moaners with lots of time on their hands to argue on mailing lists.

Chris Dillow (who incidentally writes a really great blog) has a worked example of a demand-revealing referendum which you should go and read.

Let’s try this with the systemd / TCP wrappers example. I’m going to have six voters. Four are mostly apathetic about the feature. But two of them use it, and one of those is going to have to change his whole infrastructure around if TCP wrappers goes away.

But first I have to assign a cost to this feature1. Unlike Chris’s Trident example, removing TCP wrappers from systemd is cheap. But it’s not completely free, assuming that Lennart is going to have to write some code, communicate the change, update documentation and so on. I’ll say it costs £12, which is £2/voter.

Let’s see how our six people might vote:

        Cost  Benefit
Alice   £ 2   £    5
Bob     £ 2   £   10
Charlie £ 2   £    2
Diane   £ 2   £    2
Eleanor £ 2   £  -50
Fred    £ 2   £-1000
TOTAL   £12   £-1031

Alice and Bob perceive some small benefit to the change because they think it’ll make systemd cleaner. Fred is the one who is going to have to make significant changes to his company network, and he’s not happy. Charlie and Diane are completely neutral.

The net benefits are calculated by subtracting the benefit from the cost:

        Cost  Benefit  Net benefit
                     (Benefit - Cost)
Alice   £ 2   £    5   £    3
Bob     £ 2   £   10   £    8
Charlie £ 2   £    2   £    0
Diane   £ 2   £    2   £    0
Eleanor £ 2   £  -50   £  -52
Fred    £ 2   £-1000   £-1002
TOTAL   £12   £-1031   £-1043

One thing you should notice from the “TOTAL” row is that there is no (expressed) net benefit to the change. Fred’s large negative vote has soured the whole thing. It sounds unfair that Fred is able to block this, but read on …

All we’ve done so far is asked people to guess numbers. To make it a revealed preference, we have to get people to pay real money. In this case, we’re going to ask some people to pay what is called a Clarke tax.

The tax is paid only by those who “win” (or get their way). Eleanor and Fred in this example get their way and we keep TCP wrappers in systemd. They pay the social cost of their winning that is incurred by the rest of the voters. To calculate the tax you have to remove Eleanor and Fred from the table to find the net benefit without them:

        Net benefit
Alice   £    3
Bob     £    8
Charlie £    0
Diane   £    0
TAX     £   11

Eleanor and Fred have to pay £11 in tax. (I’m unclear if this is split equally or pro-rata according to their vote). They pay this real money to Alice and Bob. Even after paying the tax, Eleanor and Fred and still better off (according to their claim). Alice and Bob have been compensated for their lost benefit.

The bids in the auction are sealed — ie. people shouldn’t be able to collude. Let’s imagine however that Alice estimated Fred’s £1000 cost and tried to neutralize it by claiming a £2000 benefit to the change. Alice would win (total net benefit becomes positive £952), but she and the other winners would have to pay a tax of £1046. This is a costly victory, but the money goes some way to compensating Fred for the changes he has to make to his company network.

1One issue with this is the estimate of the cost of the feature. I’m sure systemd developers will claim that although dropping TCP wrappers costs a bit of money in the short term, it pays dividends in the long term because of reduced code maintenance and bug reports. In other words that the cost is negative. You have to be able to provide credible costs for this to work, but I think you can express that by having the feature developers join in the voting process, in other words, revealing their true preferences as well.

Further reading

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