Hello Christian Neurohr,

Thank you for the details.

The study comprises of 8 treatment groups with 16 subjects per group and about 4-5 age groups. I wanted to make sure that all the groups are as close as possible in terms of "age".

I was wondering if there is any suitable method for this?

Thanks again,

Lalitha

Just follow the procedure of random assignment of participants and you would be fine. Here I'm pasting a paragraph on why or how random assignment is the way to go:

Random assignment is not guaranteed to control all extraneous variables across conditions. It is always possible that just by chance, the participants in one condition might turn out to be substantially older, less tired, more motivated, or less depressed on average than the participants in another condition. However, there are some reasons that this possibility is not a major concern. One is that random assignment works better than one might expect, especially for large samples. Another is that the inferential statistics that researchers use to decide whether a difference between groups reflects a difference in the population takes the “fallibility” of random assignment into account. Yet another reason is that even if random assignment does result in a confounding variable and therefore produces misleading results, this confound is likely to be detected when the experiment is replicated. The upshot is that random assignment to conditions—although not infallible in terms of controlling extraneous variables—is always considered a strength of a research design.

Source: https://opentextbc.ca/researchmethods/chapter/experimental-design/