On the assumption that the quality of reporting smoking habits is the same for ACPA positive- and ACPA negative cases (which is highly likely), and further, that the true association between smoking and ACPA negative RA is null (i.e. the observed association between smoking and ACPA negative RA is totally due to recall bias), then we can get an estimation of the maximal degree of the recall bias. Here, PRTN3 is linked to rheumatoid arthritis.