•In situ assessment of effects of office noise on cognitive performance and annoyance.•Use of sound level measurements to assess impact of background speech on workers.•Percentile level differences of sound conditions correlate with effects on workers.•No clear relationships of energy-equivalent sound levels with performance and annoyance.•Level statistics consider effects on performance and perception in occupied offices. Acoustical privacy is one of the most crucial, yet least satisfying aspects in open-plan offices. Irrelevant background speech impairs acoustic satisfaction and cognitive performance. Assessing acoustical conditions in occupied offices is challenging, and thus room acoustic parameters are commonly determined in unoccupied offices. In German speaking countries the rating level of noise is an important parameter occupational safety and health practitioners in the field often use to assess the acoustical conditions in occupied offices. The rating level denotes the energy-equivalent sound pressure level during a measurement period in an occupied office with speech sounds and takes penalties for tonal, informational and impulsive constituents into account. There is little evidence that the rating level correlates with the well-being, performance or health of office workers. As part of this study 89 different sound conditions under which subjects have to complete a number recall task and a questionnaire in laboratory conditions are evaluated with respect to their relationships with the rating level. In addition, these results are compared to percentile level statistics suggested as an alternative approach to assess the acoustical quality of office workplaces. Higher differences between the 10th and 90th percentile levels measured with fast time weighting lead to lower number recall performances and higher annoyance ratings whilst the rating level does not show any clear relationships.