However, the incomplete mailings were not influenced by patient characteristics. When comparing patients in each of the long-term follow-up groups to those without replies, there were no significant differences regarding the mRS at baseline or age at surgery, and the frequencies of the four reported comorbidities were equal. Furthermore, the prevalence of reoperation before follow-up in the 3-, 4-, 5- and 6-year groups was similar in repliers and non-repliers. Only in the 2-year group, which was closest in time relative to the majority of the reoperations, repliers had undergone fewer reoperations than non-repliers: 18% compared to 27% (p = .004). Altogether, this should indicate that there are no systematic errors in the distribution of follow-up letters, arguing for the representativity and validity of the samples.