I am trying to figure out the name for a design study in the following scenario. Imagine a hospital where patients with a certain condition (say bacteria infection) are treated with a certain medication (say antibiotic A). At some point in time, there was a decision to change the antibiotic used to treat the condition to another medication (Antibiotic B). A sample were drawn from time1 (with use of Antibiotic A) and another sample from time2 (after change to Antibiotic B). The samples can be considered independent and differences can be investigated using either Independent samples t-test or chi-square. I am not sure what type of study design this is:

The following are my thoughts so far:

  1. Causal-Comparative or Ex post facto [but since I can change the treatment (antibiotic), I am not sure this is right].
  2. Quasi-experimental - My doubt with this is that this is an observational study and the change in practice (antibiotics) was not due to experimentation but change in operation.
  3. A combination of retrospective and prospective study as stated here, however, the page and every example of this that I saw seems to suggest that this design is used on THE SAME cohort.

I saw a question about similar design here but there was no mention of the study design.

I also asked this question here and the responses seem to be between natural-experimental and quasi-experimental. However, I am not sure it is either of the two.

All assistances are welcome.