The capacity of human working memory is subject to individual differences. The number of items or chunks a person can retain in their memory may, have profound effects on a variety of other cognitive processes, such as problem solving.
I have come across a couple of ways to measure working memory such as the delayed match-to-sample task, Dual N-back task or the Wechsler Memory Scale. For the latter I have been unable to find out whether it offers an estimate of the number of items which can be held in working memory. This, however, is precisely what I am interested in. I know that the capacity measured using verbal stimuli is affected by a variety of features of the material.
Therefore, I'm wondering what are the most commonly used methods to measure an healthy individual's working memory capacity. Also, is there a reliable measure that can be easily used in online research? I am also interested in measures suitable for laboratory research, though. Finally, are abstract visual material and digits more "pure" measures than verbal ones?
I am especially interested in standardized procedures with known reliability measuring working memory and giving an estimate of the number of chunks an individual can retain.