Table 5.

Typical study weaknesses in research on the associations between COVID-19 spread and weather conditions.

Issues	Details
Independent variables	The treatments of temperature and humidity varied, including daily measurements and aggregated averages. In addition, some studies considered the lag effects, while some did not.
Dependent variables	a)Owing to the incubation period, diagnostic technique, and capability, the quality of the data on the number of positive cases and mortality rates is questionable, especially in the early stage of the pandemic. b)There is no uniform guideline for choosing the appropriate indexes (cases, mortality, cases per person, basic reproduction number, transmission rate, or others) to represent the COVID-19 spread.
Confounding variables	Most ecological studies did not control the important confounding factors related to the COVID-19 spread.
Methodologies	Although some statistical models were utilized, the comparisons between these methods/models remain unclear.
Result interpretation	Sensitivity and uncertainty analysis was required in most studies.
Exposure period	Generally, a short study period was preferred, which is not able to account for long-term trends.
Research areal unit	Results may be quite different when studies are performed at the city, provincial, national, or global scale.
Other issues	Geographical variation in viral strains; cluster infections; the role of transmission in indoor environments, etc.