Are you allowed to present data on those who declined to participate in your study?
The following is an excerpt from a study on whiplash-traumas where the authors clearly publish some basic statistics on people who declined to participate .
Emergency departments identified 6952 eligible participants, of which 3851 (55%) agreed to provide follow-up data (figure). People who participated were older (mean age 37 years vs 32 years) and more likely to be female (2133 [55%] of 3851 vs 1435 [42%] 3430) than the patients who did not participate, but not different in whiplash associated disorder grade.
 Lamb SE, Gates S, Williams M a., Williamson EM, Mt-Isa S, Withers EJ, et al. Emergency department treatments and physiotherapy for acute whiplash: A pragmatic, two-step, randomised controlled trial. Lancet [Internet]. Elsevier Ltd; 2013;381(9866):546–56. Available from: http://dx.doi.org/10.1016/S0140-6736(12)61304-X