Cohort study has investigated the effect of an asthma care support program for asthma clients under 18 years old on participants' frequency of emergency department visits. which aspect(s) of the study raise questions about the validity or reliability of the study?

The study wasn't conducted with the presence of a control group.

Explanation:

"A scientific control group is an essential part of many research designs, allowing researchers to minimize the effect of all variables except the independent variable. The control group, receiving no intervention, is used as a baseline to compare groups and assess the effect of that intervention."

The effect of an asthma care suppport program for asthma clients was studied on its participants, but there weren't study subjects without intervention as a baseline to compare the effect of the intervention.

Reference: Ezoic. “Scientific Control Group.” Explorable, 2019