The surgical theatre educational environment measures STEEM, OREEM and mini-STEEM for students (student-STEEM) comprise an up to now disregarded systematic overestimation (OE) due to inaccurate percentage calculation. The aim of the present study was to investigate the magnitude of and suggest a correction for this systematic bias. After an initial theoretical exploration of the problem, published scores were retrieved from the literature and corrected using statistical theorems. Overestimations and differences between pseudo-percentages and real percentages were plotted against real percentages. Reported STEEM overall mean score of 74.4% (pseudopercentage) was corrected to 67.9% (real percentage), eliminating thus the 6.4% OE. Corresponding figures for OREEM and student-STEEM are 73.6%, 67.0%, 6.6% and 69.1%, 61.4%, 7.7% respectively. A total of 45 overestimated scores were retrieved and corrected. OE (range 2.8 to 13.6%, mean 7.3%) showed a complete (r = -1, p < 0.001) negative linear regression of real percentages (RP), namely, OE = 20-0.2*RP. No uncorrected score can achieve less than 20%. The non-0-based 1-to-5 coding overestimates STEEM, OREEM and student-STEEM educational environment scores if expressed as percentages due to the "1-to-5 bias", or rather 1-to-L bias, whereupon L correlates to the number of points in the Likert scale, the number of options. The worse the educational environment the greater the overestimation, reducing instruments' usefulness exactly then when alarm bells should be ringing. Hence, question coding should always be zero (0) based, as proposed by Likert. The 1-to-L bias applies to any questionnaire at any field of research.