This invention concerns a method for measuring cognitive load of a person in performing a task. In other aspects the invention can be expressed as a computer and as software that are used to perform the method. The computer (50) has an interface having a plural number of unimodal input (20, 30 to 40) and output devices, and a cognitive load analyzer (50). The analyzer comprises a receiver to receive input data signal streams (25, 35 and 45) from respective devices (20, 30 and 40). A classifier (56 to 59), (66 to 69) and (76 to 79) is also provided to identify predetermined “meta-interaction patterns” from the streams (25, 35 and 45), and to weight the identified predetermined “meta-interaction patterns” to produce respective weighted outputs. A combiner (80) to fuse the outputs to produce a measure indicating the person's cognitive load.