In the survey listing page in admin panel, when a Survey is closed ( tokens exist), there is a Statistic available, called response rate.
This rate measures the percentage of completed tokens to the sum of them.
Many survey creators need to have a different statistic, one that is best called redeem rate.
The redeem rate should be the percentage of Completed and started (but abandoned) tokens to the sum of the tokens.

This is exactly what is stated in the http://ideas.limesurvey.org/ideatorrent/idea/425 issue in the ideas forum
In order to do so we should be able to distinguish three different states for a record in the participant table
0-Not started survey with this token
1-Started survey with this token
2-Completed survey with this token

This statistic cannot be calculated right now, when a survey is anonymized

Our proposal is to allow for the 'Completed' property in tokens_dynamic model to have thee accepted values.
Y - When the token is completed
N - When the token is not started
I - When the token is started but the response was left incomplete (and token is still valid for use)
The value of "I" in the completed property might easily be set the moment the row in the response table is created
In that way we wont change any of the functionality that exists right now and we can provide with this new statistic which in cases is of paramount importance.