A possible measure of voter choice is the information-theoretic value of the
result of an election, which appears to be new. For instance, in a
dictatorship which has mock elections, the result is known beforehand, and
therefore the information-theoretic value is zero. On the other hand, if the
electorate is given a choice between three candidates then, assuming that
each outcome is equally likely, the information-theoretic value is
log_{2}(3)=1.58. As the number of possible outcomes increases, so
does this measure of voter choice.

For values of the Rose Index, Kestelman gives values for the major electoral systems. It must be acknowledged that the Rose Index as a measure of party proportionality, may not be appropriate for STV elections, as pointed out by David Hill.

We compute the values for a hypothetical election for a 600 seat assembly in
which there are three parties. For the use of STV, we take 120
constituencies each electing 5 members. For the regional list, we take 10
regions electing 60 candidates each. For the additional member system, we
assume 300 seats elected directly and 300 added by proportionality. Note
that if *n* seats are to be filled with 3 parties, then the number of
ways to do this is *n*^{2}/2+3*n*/2+1. We assume that all
possible outcomes are equally likely. The entries in the diagram are as
follows:

- First Past The Post (
**FPTP)**: Rose Index 70% (UK), voter choice is600 log

_{2}(3)=951. - Alternative Vote (
**AV**): Rose Index 72% (Australia), voter choice is600 log

_{2}(3)=951. - Single Transferable Vote (
**STV**): Rose Index 92% (Ireland), voter choice is120 log

_{2}(^{5}C_{15})=1386.(We are assuming each party has five candidates and therefore could theoretically obtain all five seats; hence the number of possibilities is the number of ways of selecting 5 from 15.)

- Additional Member System (
**AMS**): Rose Index 98% (estimated), voter choice300 log

_{2}(3)+log_{2}(300^{2}/2 +3×300/2+1)=491. - Party List (
**PL**): Rose Index 98% (estimated), voter choice islog

_{2}(600^{2}/2+3×600/2+1)=17.5. - Regional party Lists
**(RL**): Rose Index 98% (estimated), voter choice is10 log

_{2}(60^{2}/2+3×60/2+1)=109.

The conclusion from this diagram is hardly unexpected: party lists do not give voter choice, and FPTP/AV do not give party proportionality, while STV can claim, to a reasonable degree, to provide both.

- P Kestelman. Is STV a form of PR?
*Voting matters*. Issue 6. p5-9. - I D Hill. Measuring proportionality.
*Voting matters*. Issue 8. p7-8.