Small and limited sample size
The relatively small sample of origin countries (32 for Denmark, 11 for Finland) raises concerns about the study's generalizability. This limited sample size also makes it difficult to create reliable estimates and conclusions. It is also important to note that two origin groups from the Finnish data were not represented in the Danish sample, reducing the number of comparison cases to nine.
Correlation does not equal causation
The study is heavily dependent on correlation, and while it posits a causal relationship, the findings cannot confirm a causal link between national IQ/Muslim percentage and immigrant fiscal contributions. It is also important to note that many potential confounders such as selection effects, language barriers, length of stay in the host country and cultural differences were not measured or controlled for in the models.
Inconsistencies in data collection
The data for both countries were collected using slightly different methodologies which differ with respect to sample parameters. The Finnish estimates only considered the working-age population (20-62), while the Danish numbers included the entire population. This inconsistency could confound comparisons between the two datasets.
Missing variables and weak causal inference
The paper focuses on the correlation between national IQ, percentage Muslim and fiscal contribution. It is not clear from the paper whether the model used controlled for variables such as migrant's education level and host country social welfare system. The author mentions that individual-level data on religion, values and cultural practices were not available. This weakens causal inference that can be drawn from the models.