As this was not a controlled/randomized experiment (we never declared it as such), we did not seek to experimentally manipulate affects. Besides that recent studies have questioned the effects of mood-induction techniques (e.g., Forgeard, 2011, referenced in the article), we wanted to study the participants’ natural affects. Additionally, a revision of this document declared the study as a natural experiment (i.e., observational studies not controlled like in randomized experiments). However, the editor suggested that there was no need to call this a natural experiment because it appeared like a textbook-related term. We decided to follow his suggestion to employ the more generic term study or experiment throughout the text. Still, controlled experiments might be useful, and we call for them in the "Implications and future research" section.
To answer the second question: yes, the study is based on 42 participants. In the future studies there should be a larger sample size as also suggested in the article.