Objectives To evaluate the difference between chronological and random sequence reading in a series of radiographs with 11 years’ follow-up. In addition, the influence of the starting point and length of series was evaluated.

Methods Two experienced readers independently and repeatedly scored digitised radiographs of 62 patients at time points 0, 2, 5, 8 and 11 years of follow-up from the COBRA follow-up database according to the Sharp/van der Heijde method. A linear mixed model was fitted to the data.

Results Over 11 years the mean scores increased by 3.8 points per year. Compared to random reading, chronological reading resulted in a slightly increased progression rate of 0.4 points per year (p=0.008) and a lower standard error of the mean total progression rate of 0.30 (compared to 0.35 for random reading). Over 11 years, this results in a small difference in progression estimates of about five points, but a highly relevant difference of over 25% of patients needed in a study to find a difference in radiological outcome between two groups. Reading of short series, or series including a baseline radiograph, results in a significantly higher yearly progression rate compared to reading of long series, or series not including a baseline measurement.

Conclusions Chronological reading of radiographs is preferred above random reading, due to decreased variability around the estimation of the progression rate; this increased efficiency translates into smaller sample sizes, or increased power to detect small differences. For studies with long-term follow-up, the same two readers should read all radiographs, including baseline.