I have not had any personal experience with this particular type of written assessment method, but I think it is quite likely the firm is looking to assess the same key competencies: time-management and resilience under pressure, ability to identify and summarize key issues in large bodies of text, use of formal language, commercial awareness and problem solving skills. As such, I do not think your preparation for this type of written assessment format should be very different from your preparation for an exercise with a more common format.
The only things I can think of that you may want to consider as altering in your preparation are the following:
- Broader scope: An article will normally treat a broader topic and will aim to be more comprehensive than an email relating to a particular task. As such, when preparing I would look for written exercises asking me to summarize/comment on a wider set of issues. However, a corollary of that may be that you are not expected to go into as much depth in any of the particular issues.
- Length: Because of the broader scope, an article may generally be longer than an email.
- Impersonal and objective tone: As opposed to an email, which will relate to a particular issue a client cares about, an article is generally supposed to be more objective and impartial. You may therefore have to consider the perspective of stakeholders other than the firm's clients and to avoid the temptation of analysing the issues from a 'client-centric' perspective. However, if you are asked to also make an argument and give your personal view in the article (rather than an impartial commentary), definitely avoid sitting on the fence.
Finally, I would advise you to try to gather more information about the details of the written task at this specific firm. Resources like the
TCLA Interview Database may come in very handy.