In addition, no significant effect modification by time since diagnosis was observed (P interaction=0.34).
To assess possible effect modification by time of operation, the analyses were also performed stratified by time periods.
As such, this model was not significant for heart rate (i.e., no effect modification by time during exposures).
None of the other three factors that were tested against effect modification by time period reached significance.
While Thornton et al did not find a significant effect modification by employment status, our study found an effect modification by time spent at home, which may be due to the technique used to estimate residential exposure.
The effects of the last four explanatory variables were also tested one-by-one against modification by time period at the more stringent significance level of 0.01 than the level of 0.05 used otherwise.
To test whether the associations between wheezing, atopic dermatitis and functional constipation were different at the age of 24, 36 and 48 months, the effect modification by time was evaluated by adding the product-term of wheezing/atopic dermatitis and time (eg, wheezing×time) as an independent variable to the crude GEE model.
The effect modification by time between sampling and end-stage renal disease diagnosis on the associations between Cd, Pb, and Hg concentrations and end-stage renal disease was further investigated within subsets of time between sampling and end-stage renal disease defined by quartile limits (See Additional file 1: Figure S3).
Effect modification by age, time and exposure rate was present in the radon-induced lung cancer risk.
To assess possible effect modification by calendar time, we considered models that ended the cohort follow-up (and hence the analysis) in varying calendar years, in 3-year intervals back to 1987 (before peak exposure).
