A cognitive psychologist investigated the effectiveness of a new mnemonic visualization technique ("Method of Loci") on foreign language vocabulary acquisition using two separate groups of adult learners:

• Group 1: One group of learners spent 15 minutes training in the "Method of Loci" before being given 5 minutes to memorise a list of 30 Swahili words.
• Group 2: A different group of learners spent 15 minutes resting quietly before being given 5 minutes to memorise the same list of 30 Swahili words.

Afterwards, the researcher calculated the mean number of words correctly recalled (out of 30) for each group, as shown in Table 1.

Table 1: Mean vocabulary recall scores (out of 30) for Group 1 and Group 2

The researcher concluded that the mnemonic training may not have been the sole cause of the difference in mean recall scores between Group 1 and Group 2. The researcher was concerned that individual differences in baseline memory capacity and linguistic ability among the participants in the two groups confounded the results.