December 1966 – vol. 59, issue 12–558 Week-end bias, length of stay of rarities and proportion of rarities undetected Sirs,--Although the effect of 'week-end bias' (the increased watching at week-ends) is often commented on, there appear to be no figures to show exactly how marked it is. In an attempt to determine this, I have analysed the annual 'Reports on rare birds . . .' for the eight years 1958-65 (Brit. Birds, 53: 153-173, 4Q9-431;... This article is only available to registered users.To read more, please log in or purchase a subscription.