Statistics
REGRESSION, HEAT
Stata 16.1 (StataCorp, College Station, Texas, USA) and Prism 9.1 for macOS (GraphPad Software, La Jolla California, USA) were used for statistical analysis and graphical presentation. Heat maps were created in Microsoft Excel for Mac 16.43 (Microsoft corporation, Redmond, Washington, USA). Participant clinical features, eNO, weather and air pollutants are presented as medians with minimum and maximum values for continuous variables and frequency with percentages for categorical variables. Differences between infant groups were tested with Mann–Whitney two-tailed test for continuous variables and Fisher’s exact test for categorical variables. eNO50 concentrations were right skewed with the mean greater than the median. To achieve a near normal distribution, these values were transformed to their square root (eNOSqR).To test for seasonality of eNO related to mean daily temperature, the months of the year were categorized by mean daily temperatures above or below 20 degrees Celsius, with November to March being above and April to October being less than 20 degrees. Differences between the groups (mean daily temperature above or below 20 degrees Celsius) were tested with the Mann–Whitney two-tailed test for non-normally distributed data. Due to increasing seasonal variation in eNO results, month of test was parameterized with a sine model “sinterm” (assumed value of zero at June, peak in March, and trough in September with increasing amplitude for each year) for inclusion in subsequent regression analysis of eNO. To test for seasonal variation in air pollutants, month of test was parameterized with a cosine model (assumed value of zero in March and September, trough in June, and peak in December).A ‘least absolute shrinkage and selection operator’ (LASSO) approach was used to reduce the dimensionality of the air pollution data and estimate model coefficients. Lambda was determined based on tenfold cross-validation. Seventeen variables of interest were included in the LASSO for eNO: eight 24-h mean to 12 pm air pollutants (NO, NOSpearman’s rank correlation coefficients were calculated for eNO, sinterm and seven-day mean air pollutants, as well as by season with a heat map generated for the same variables.As there is limited data on the association between eNO and recent (seven days or less) postnatal environmental air pollution exposure in young infants, a power calculation could not be performed prior to the study.