Do shorter stated survey length and inclusion of a QR code in an invitation letter lead to better response rates?

Invitation letters to web surveys often contain information on how long it will take to complete a web survey. When the stated length in an invitation of a survey is short, it could help to convince respondents to participate in the survey. When it is long respondents may choose not to participate, and when the actual length is longer than the stated length there may be a risk of dropout.
This paper reports on an Randomised Control Trial (RCT) conducted in a cross-sectional survey conducted in the Netherlands. The RCT included different version of the stated length of a survey and inclusion of a Quick Response (QR) code as ways to communicate to potential respondents that the survey was short or not.
Results from the RCT show that there are no effects of the stated length on actual participation in the survey, nor do we find an effect on dropout. We do however find that inclusion of a QR code leads respondents to be more likely to use a smartphone, and find some evidence for a different composition of our respondent sample in terms of age.