If I read a medical paper that concluded and headlined that “cancer kills left-handed patients” but studied only left-handed patients, I might think the study was technically correct but still reasonably wonder if the left-handedness is relevant to the study’s results.

Entering the parenting field, I read a lot of studies which only involved women. It seemed reasonable: if you have a child, there's almost always a female parent that can be asked to include in the study, and women receive/need maternity leave so can more easily participate in studies especially those that start in the hospital itself.

While reading the studies, I had to assume that they were applicable to men too but I didn't know for sure.

More recently, in the EU, they've been replicating those same studies using fathers (e.g. parental attachment to newborns following skin-to-skin), and finding out that yes the results hold true for men.

It's important to include both genders, but lacking that... we can assume the same effects are true.

> wonder if the left-handedness is relevant to the study's results

Considering this in generic, such distinctions are relevant to all scientific research studies for two reasons:

First, the hypothesis was only tested for a population of left-handed patients. So, while it serves as a foundational basis for future research both exclusively on left-handed patients, and for research on populations exclusive of left-handed patients, it cannot serve as a basis for any claims about any population exclusive of left-handed patients. This is generally applicable to all scientific research: you must not draw conclusions that exceed the scope of your study.

Second, there is no 'best' level of population diversity for studies to target in their research. One study may target left-handedness; another may target blonde six-fingered men; and a third may target an even distribution of people from all ages and races, but restricting to one region's current population alone in that selection. Restrictions are normal and correct to consider, to implement, and/or to acknowledge, so long as the restrictions do not manipulate the outcome.

Moving on to trying to understand your specific concerns regarding this specific study, I still don't see any opinions expressed by you about this study. You're describing the basic circumstances about scientific research but have not yet made any claims, nor have expressed any personal opinions, about this study in particular and why you're interested in talking about it. There are no sentences that yet contain "I think that [opinion]" or any synonym thereof, and so no takeaway is possible from your comments thus far.

Perhaps one of these examples will help you find a way to communicate your views to us. These are just creative napkin sketches, to help encourage you begin sharing your viewpoint with us. You could tackle them as direct questions and answer at length — a simple yes/no will fully devalue your views, so I discourage that! — or you could construct your own opinions as statements rather than questions and then say "I feel / believe / think that ___"; whatever works best for you.

1. Is your argument that this study was inappropriately selective, and that should invalidate its results as inapplicable to all populations?

2. Are you appreciative of the study's design and outcome, and wish to see it repeated for another population?

3. Do you have any conflicts of interest to declare regarding the population restrictions of this study?