The use of mobile telephones in experience sampling studies makes it possible to measure psychological constructs in the lives of study participants outside of experimental laboratories. Up to now, the method of experience sampling, and especially mobile phones as survey instruments, have been used mainly to measure self-reported variables and movement data (Kwok, R., Nature 458 (2009), 959-961). We show that cognitive performance can also be measured using mobile telephones during participants‟ everyday activities. To this end, we developed a mobile phone version of a working memory updating task that has been proven to be a reliable and valid indicator of working memory capacity in experimental and psychometric studies. In this task, participants have to commit several single-digit numbers to memory and then carry out simple arithmetic operations using the various numbers.

Our survey data on 378 adults aged 14-86 who carried out this task by mobile phone during their everyday activities on 9 days at a total of 54 different points in time show that this task allows for efficient and reliable measurement of working memory performance. A comparison with data from a laboratory study in which young and older adults completed the same task by computer on 100 days in a controlled laboratory environment shows evidence of stronger fluctuations in performance in an everyday context than under standardized laboratory conditions. This underscores the importance of studying psychological constructs more outside laboratory conditions, in the everyday lives of study participants.