Just wanna start saying that hope everyone is safe and healthy. I have thought about user testing done for apps sharing the same domain to start the design improvement process. For example I have had 4 clients last year all into retail banking domain with very similar features. I just wanted to hear your thoughts on conducting the user testing each time with the same purposes for each app (purpose is to detect usability problems). Considering the cost of testing (time and budget) I believe with the know-how in hand expert review is also sufficient in this case. I ended up with detecting the same problems I've detected through expert review and feel like I'm just spending unnecessary resources and extending the timeline of the project. What do you think on this matter? How would you communicate it with your client?
Interesting topic. Basically, I think you are right on a general design topic and most usability topics. But on different target groups (even within a domain) you should be careful.
An interesting thought on this is the look into some of the bigger companies facing this issue. There a new topic arises insight repositories. This is a location where all insights from former user and usability testings are stored and can be used to back newer decisions. The main benefits are fewer resources, faster turnaround, and better communication/availability of these insights to all stakeholders (management, dev, UX, UI, concept, etc.) of the team. Tools via google: https://www.google.com/search?q=user+Research+repository
In your case, this won't work precisely because you are not allowed to show one customer the results of a former study of another client. In the end, it's not about trust in your expert skills but the urge to test it with the very own characteristics of this group. They don't want to see their head rolling by "trusting" an expert over the user. (Especially if there is a user-centered directive in this project.
Thus my personal summary would be: You can't show you client "real" testing data, thus you should do testing. On the other hand: Couldn't you do the mockup with your expert knowledge and then refine this to a new level via additional research? Win-Win :-)
It depends on how much the features and users differ per product, and what the risk and impact will be when you don't test. But if an expert review shows that the cost of user testing most likely won't outweigh the cost of possible fixes afterward, it's fair to assume that risk and impact are low. You could communicate past test results, the expert review and an after-sales plan based on a risk, impact and costs analysis.